Gemara

40b (Beraisa): If a garment is big enough to cover Rosho v'Rubo (the head and majority of the body) of a child, and an adult goes out with it Arai (occasionally), it requires Tzitzis;

If it is not big enough to cover a child (in this entire Halachah, this refers to Rosho v'Rubo), even if an adult goes out with it Arai, it is exempt from Tzitzis.

The same applies to Kilayim.

Suggestion: This means that the same applies to the Isur of Kilayim.

Rejection (Mishnah): Arai does not apply to Kilayim. (Even if one would not wear the garment Arai, it is forbidden.)

Rishonim

Rambam (Hilchos Tzitzis 3:1): The Torah obligates putting Tzitzis on a garment big enough to cover a child who goes alone in the market and does not need someone to guard him and go with him.

Ra'avad: This is correct. The Gemara does not refer to any child, for it seems that adult goes out with it Arai.

Nimukei Yosef (Hilchos Tzitzis 11b DH Talis): If a child, who is ashamed to go outside to the market uncovered, can cover himself with a garment, it is obligated. It is called 'Kesuso.'

Mordechai (Perek ha'Techeles 943, citing R. Shimshon): My Rebbi said that Talesim with a neck opening, half in front and half in back, are not ideal. "Asher Techaseh Bo" somewhat connotes also the head. We require a garment big enough to cover the head of a child, and an adult goes out with it Arai. I.e. it must be considered a garment proper to cover at least a child. We bless 'Lehis'atef (to wrap oneself) b'Tzitzis.' The Aruch obligates a turban, i.e. one that covers the head and hangs down a lot. One can wrap himself in it. My Rebbi denounced those who make it (with or) without Tzitzis. He was unsure if it is obligated.

Maharam Mintz (110 DH Lo): In our lands, no one wears a Talis after leaving the Beis ha'Keneses. It is better to wear a Talis Katan than to go without Tzitzis at all.

Darchei Moshe (8:3): Rishonim were lenient to allow us to wear a Talis Katan without Atifah (wrapping). They never intended to permit less than the Shi'ur for a Talis! However, the Mordechai connotes that the custom was that it is smaller than the Shi'ur. Meticulous people bless on a Talis Gadol, and also exempt the Talis Katan that they wear the entire day. Most people bless on the Talis Katan, even though it is too small.

Poskim

Shulchan Aruch (OC 16:1): The Shi'ur to obligate putting Tzitzis on a Talis is length and width enough to cover Rosho v'Rubo of a child old enough to go in the market by himself without someone to guard him.

Beis Yosef (DH Shi'ur): Rashi explains that an adult would wear it Arai and not be ashamed to go to the market. The Tur says that it must cover a nine year-old. This is unlike the Rambam, for children less than nine go alone in the market. Perhaps the Tur explains the Rambam like Semag says, that he goes alone with his Talis, and one need not guard him lest he lose his Talis, and this is when he is nine.

Bach (DH u'Mah): The Tur learned the Shi'ur of nine years from the Rambam; this is the age to go in the market alone. Semak cites an opinion that the child reached the age of Chinuch, i.e. six or seven. He is obligated in Tzitzis (Sukah 42a). We hold like the Tur. The Maharal cited Semak to say that a child at the age of Chinuch is obligated only if an adult would wear the garment Arai. This is a terrible mistake based on a printing error!

Beis Yosef (DH Kasuv): Orchos Chayim says 'enough to cover the head and majority of a child' means the length and width. Mahari Avuhav says that 'an adult goes out with it Arai' explains the Chiyuv. It is obligated because an adult goes out with it Arai, but not because it covers a child, for a minor is exempt. This is why the Beraisa taught 'if it is not big enough to cover a child...', but did not teach 'if it is not big enough for an adult to wear Arai', because an adult going out with it is not a Tanai (condition) to be obligated. (It only explains when the Chiyuv applies.) The Rambam and Rav Amram Gaon mentioned only enough to cover a child.

Beis Yosef (DH v'Kach, citing Mahari ben Chaviv): The Tur bergan with the Rambam's words and concluded with the Beraisa. This gives room to err and think that it depends on an adult going out with it Arai. Really, the Beraisa teaches when an adult who wears a small garment is liable, i.e. if it is enough to cover a child. The Nimukei Yosef explained succinctly that it is obligated when it is called 'his garment', and exempt when it is not called 'his garment.' The Rambam wrote the Beraisa concisely, based on its intent. Therefore, he did not mention an adult.

Beis Yosef (ibid.): Talmidim should learn from here not to delve into the words of a Posek before seeing the source!

Bach (1): The Tur connotes that an adult who goes out with it Arai is obligated only if it is big enough to cover a child, but if he goes out with it regularly, he is obligated even if it does not cover a child! The Beraisa connotes like this. The Seifa says 'if it is not big enough to cover a child, even if an adult goes out with it Arai, it is exempt.' It does not say 'if an adult goes out with it' Stam, i.e. regularly! It also connotes that even if it covers a child, if an adult is ashamed to go out with it in the market even Arai, it is exempt even if he wears it in the house. The law also depends on the adult. Why did Mahari Avuhav, Mahari ben Chaviv and the Beis Yosef say that if it covers a child, surely an adult would go out with it Arai. There are two Tanayim, like the Tur says.

Taz (1): The Rambam and Shulchan Aruch hold that the Beraisa mentioned an adult for the Chidush that if it covers a child, even if an adult goes out with it Arai, it is obligated. Surely, if adult goes out with it regularly, it does not depend on children. Surely it covers a child! The Tur holds that there are two Tanayim. The Seifa omitted 'if an adult does not go out with it Arai', for a Kal va'Chomer teaches this. The primary Chiyuv is due to an adult, just we measure based on a child. Even though it is an Arai garment for an adult, it is exempt (if it does not cover a child). All the more so, if it is not a garment at all for an adult, it does not matter if it covers a child! Why did the Tana give two Shi'urim, since they are really one? Not all children are the same. The Tur mentioned a nine year-old. Not all nine year-olds are the same! Adults differ about whether they would be ashamed to go with a small garment! Therefore, both were taught together.

Mishbetzos Zahav: The Tana taught also about adults, so we will know which children he discusses.

Taz (1): The Tur learned from Gitin 59a, which says that Pa'utos (mature children) are nine or 10 years old; it depends on their sharpness. Our Beraisa discusses a child who goes among people to do business, i.e. from nine and above. The Rema does not mention an adult for the Shi'ur, rather, to teach that the Chiyuv is only on adults. However, the primary Shi'ur is based on children.

Note: I do not understand how we learn nine years from Gitin 59a. Pa'utos can be from six years, if they are sharp!

Magen Avraham (1): The Tur does not argue with Semak. Tosfos (Erchin 2b DH she'Hegi'a) says that nine is the age of Chinuch.

Eshel Avraham: The Tur holds like the opinion in Tosfos that nine is the age of Chinuch for all Mitzvos. (Tosfos rejected that opinion.)

Rema: Then, it is obligated. This is only if an adult would wear it Arai and go outside to the market.

Magen Avraham (2): What is the Rema's Chidush? He teaches that it depends also that an adult goes out with it Arai, like the Tur says. However, in Darchei Moshe he said like the Beis Yosef. This requires investigation. The Gemara supports the Bach, who exempts even in the house if an adult would not go out with it Arai in the market, even if it covers a child. According to the Beis Yosef, why did the Gemara mention an adult going to the market? The Rambam omitted this, for he holds that Stam, if it covers a child, an adult would not be ashamed to go out with it Arai in the market. The Ra'avad connotes like this. One should be stringent not to bless if one would be ashamed to go out with it Arai in the market. If one would wear a Talis that others would not, his opinion is Batel to that of normal people.

Gra (DH v'Davka): Also, if it does not depend on an adult going out with it Arai (just this is when the Chiyuv applies), how can we explain the Beraisa according to the opinion that Talis is a Chiyuv on the garment? (He obligates even when it is not being worn!) Also, what was difficult from the Mishnah 'Arai does not apply to Kilayim'? We must say that the Rambam had a different text. From it, he learned that it depends on a child going alone. The Tur and Rema explain like Rashi, unlike Mahari ben Chaviv.

Magen Avraham (Reish ha'Siman): One should make the shoulders (on the side of the neck-opening) wide, lest they be Batel to the air on both sides of them.

Mishnah Berurah (1): They should be wide, so they are recognizable and considered a garment, and not straps.

Mishnah Berurah (2): If adults normally go out with it in the market without shame, the Bach connotes that it is obligated even if it is not big enough to cover a child.

Mishnah Berurah (3): 'Adult' refers to a 13-year old.

Mishnah Berurah (4): Going out the doorway to the street is also like going in the market. Poskim disagree about a garment that covers a child, but an adult is ashamed to go out with it in the market. Acharonim are stringent not to bless on it. Pri Etz Chayim says that the Shi'ur to cover a child is 3/4 Amah long and half an Amah wide. Others say that there is no support for this, especially regarding the width. Pious people require an Amah in front and in back, excluding the neck opening. One may bless on this without any Safek. At least it should be 3/4 Amah on every side to avoid a Safek Berachah l'Vatalah. If a garment had a Shi'ur, but it folded and now it lacks a Shi'ur, since it is destined to spread out, it is as if it is spread out. If due to stitching it does not have a Shi'ur, it is exempt.

Bi'ur Halachah (DH v'Az): If a boy is six, mid'Rabanan his father must train him in the Mitzvah of Tzitzis. The garment must cover the boy's Rosho v'Rubo. If one is not ashamed to wear a garment, Arai or regularly, but others would be, his opinion in Batel. Surely we say so if the garment does not cover a child. The Bach discussed one who regularly wears it like a garment outside without shame, but not one who tucks in the corners lest they be seen. He is ashamed to go out with it like a garment!

Bi'ur Halachah (DH l'Shuk): How is one Yotzei with the Talis Katan we wear nowadays? An adult would not go out with it in the market! It suffices that a 13year old would go out the doorway with it in summer. Also, it is exempt only if one would be ashamed because it is too small. Surely, if a Talis Katan covers an adult to his ankles, it is obligated even if he is ashamed to go out with it in the market because others go out with it under their clothes! If not, even a Talis Gadol would be exempt where people do not wear it outside! The Pri Megadim says that the Bach forbids blessing on a disgraceful garment even if it is big. This is astounding. If so, it depends on every person! The Beraisa did not say that he goes in the market with it, rather, an adult goes in it!

Kaf ha'Chayim (2): Chesed l'Alafim says that if it is less than an Amah of five Tefachim, one may not bless on it or wear it in Reshus ha'Rabim on Shabbos. It is better to wear it than to go the entire day without Tzitzis; he intends for it when blessing on the Talis Gadol. The Ari Zal says that Tzitzis is 'Ohr Makif' (encompassing light). The Shi'ur of a Talis Katan is to cover the entire body, front and back, from the neck until the end of the chest, i.e. the bottom of the ribs in the middle, without the head and arms. The Shi'ur of aTalis Gadol is to cover also the head and arms. It varies for each person, based on his size. Lev Chayim requires one and a half Amos long, and also wide. This suffices even for a big person. One should measure for himself. If he can connect the front and back under his elbows, this suffices for the width. An average person can rely on 1.25-1.3 Amos. One with Yir'as Shamayim will make it bigger so he can wrap himself and bless Lehis'atef b'Tzitzis.

Kaf ha'Chayim (2): Some say that extra length can compensate for deficient width, and vice-versa. Others, including the Ari Zal, disagree. One should make the neck opening as small as possible, or ensure that the chest is covered.