Gemara

(Beraisa): (The middle finger is used to measure) an Amah.

Eruvin 4a (Rava citing Rav Nachman): All Amos (that Chachamim taught) are six Tefachim. Some (e.g. of Kil'ayim) are 'smiling' (the Tefach (hand-width) is measured with space between the fingers), and some (e.g. of Mavoy and Sukah) are 'sad' (without spaces).

45a (Mishnah - R. Yochanan ben Nuri): If one slept on the road (on Erev Shabbos) and did not realize that it was dark, he has 2000 Amos in every direction;

Chachamim say, he has only four Amos;

R. Eliezer says, he is in the middle of his four Amos;

R. Yehudah says, he may go four Amos in any direction he wants;

48a (Rava): They argue about where he may walk, but all agree that he may carry only within four Amos.

(Beraisa - R. Meir): "Shevu Ish Tachtav" - one must stay in an area equal to what is under himself.

His body is three Amos. We add one Amah for him to stretch his arms and legs.

R. Yehudah says, his body is three Amos. We add one Amah for him to take something from 'under' (next to) his feet (when he lies down) and put it 'above' his head.

Question: What do they argue about?

Answer: They argue about whether he gets slightly more than four Amos, or (only) precisely four Amos.

Rav Mesharshiya (to his son): Ask Rav Papa if this is based on the person's own Amah (the distance from his elbow to the end of his middle finger), or on the Amah of Hekdesh (six Tefachim; it is the same for everyone);

If he tells you that he gets four Amos of Hekdesh, ask how Og Melech ha'Bashan could manage!

If he tells you that he gets his own Amos, ask why it was not taught in the Mishnah that lists things that depend on the person himself!

(Rav Papa): He gets his own Amos. It was not taught in the Mishnah because it is not always true. One with small limbs (but average height) receives Amos of Hekdesh.

Shabbos 96b - Question: What is the source that one who throws four Amos on the ground is liable?

Answer: A tradition from Moshe from Sinai teaches all the Chiyuvim for four Amos in Reshus ha'Rabim.

Rishonim

Rif and Rosh (Eruvin 13a and 4:11): R. Meir allows precisely four Amos. R. Yehudah allows slightly more.

Rambam (Hilchos Shabbos 12:15): One may carry in Reshus ha'Rabim within four by four Amos at his side. He may carry within this entire square. We measure based on his Amah. If his limbs are undersized, he gets four Amos of an average person. Tradition teaches that "Shevu Ish Tachtav" forbids carrying outside this square, which is the length of a man when he spreads his hands and feet.

Magid Mishneh: Some, including the Rashba, say that we forbid carrying at once from one end of the eight Amos to the other end, but one may carry within the eight Amos, as long as he does not carrty more than four Amos at once. Perhaps the Rambam agrees. He did not say that the four Amos are only in one direction. However, in Perek 27, the Rambam says that the place of a person (who left the Techum, i.e. 2000 Amos from the end of the city) is four Amos in one direction, like R. Yehudah. It seems that the Rambam means the same here. This is like the Rif, who did not mention eight Amos.

Rashi (11a DH Zu Amah): All Amos of Hekdesh are measured to the end of the middle finger, since it is the longest.

Poskim

Shulchan Aruch (OC 349:1): Anyone may carry in Reshus ha'Rabim within four Amos. We measure based on his Amah. If his Amah is small, he gets four Amos of an average person, each of which is six Tefachim.

Beis Yosef (DH u'Mah): The Tur allows spacious Amos. Rashi explains that R. Meir allows spacious Amos, and R. Yehudah allows precisely four Amos. The Rif and Rosh explain oppositely. The Tur holds like them, for the Halachah follows R. Yehudah. It seems that R. Yehudah argues only when we estimate based on one's own Amos, so he need not be cramped. When we measure based on an average person, surely he gets precisely four, like we say that the Amah of six Tefachim was sad.

Rebuttal (Bach 2): No one before the Tur said that when we measure based on an average person, one gets precisely four Amos. If the Tur was Mechadesh this, he should have said so explicitly! Rather, first the Tur taught that one gets Amos of his size. Then he wrote that he gets spacious Amos. Then he wrote that a small-limbed person gets average Amos, but surely also they are spacious. R. Meir and R. Yehudah argued Stam about this, without distinguishing for different people. On 4a, the Gemara discussed Stam Amos, but not when they were specified. The Shulchan Aruch follows the Rambam; the Rema (396:permits spacious Amos, like the Tur. Even if you will say that it is a Safek whether R. Yehudah is stringent (like Rashi explain) or lenient (like the Rif and Rosh), we follow the lenient opinion in Eruvin.

Beis Yosef (ibid.): The Rambam did not specify whether they are smiling or sad. He allows four Amos, which is 'the length of a man when he spreads his hands and feet.' It seems that he holds like R. Meir. Why does he rule like him against R. Yehudah? Perhaps he holds 'his length when he spreads his hands and feet' is precisely four Amos, and he explains R. Yehudah the way the Rif and Rosh explain. He holds that since we conclude that we are stringent to use sad Tefachim, likewise the four Amos are precise, unlike R. Yehudah.

Magen Avraham (1): The four Amos are precise.

Mishbetzos Zahav (1): A tradition teaches about the Isur to pass or throw four Amos in Reshus ha'Rabim. A large person gets bigger Amos to transfer within four of his Amos, but to transfer at once, everyone gets average Amos.

Rebuttal (Sha'ar ha'Tziyon 3): R. Yehudah explained his opinion that it is in order that one can take an item below his feet and put it under his head. We conclude that we use a person's own Amos, so that the verse applies to everyone. This shows that one may transfer four of his own Amos at once! R. Meir did not argue about this, only about whether they are precise Amos or spacious Amos! This requires investigation.

Defense (Chazon Ish 112:11 DH v'Ha): One receives his Amah regarding taking something outside his four Amos. A giant could not manage without this. To pass something in Reshus ha'Rabim, we measure from where it was uprooted to where it was set down. This depends on Amos of Hekdesh. There is no problem to say that this is even for giants. It is a tradition from Sinai, like it says in Shabbos 96b. This is why the Gemara asked only here about the size of the Amah, and not whenever it discusses transferring four Amos in Reshus ha'Rabim.

Magen Avraham (3): If a big person has a small Amah, four (of his) Amos are not enough for him.

Mishnah Berurah (4): If an average height person has a small Amah, four (of his) Amos are not enough for him, therefore, he gets average Amos.

Sha'ar ha'Tziyon (4): This is like Rashi, R. Chananel and R. Yerucham. This implies that if he was also short, he gets only four of his Amos, even though they are smaller than average. The Graz says so.

Note: If he is taller than average, also four average Amos are not enough for him!

Rema: See Siman 396.

Mishnah Berurah (5): There, regarding Techumim, the Rema is lenient to give spacious Amos. Here, regarding carrying four Amos in Reshus ha'Rabim, one should be stringent to allow only precise Amos.

Kaf ha'Chayim (7): Eliyahu Rabah says that the Rema and Levush did not mention spacious Amos here, because here, regarding carrying, we are stringent.

Kaf ha'Chayim (1): Toseses Shabbos says that we hold like the Tur, who gives spacious Amos. The Acharonim disagree.

Kaf ha'Chayim (2): One may not move an item more than four Amos, even if he stops in the middle, like it says in Sa'if 5.

Chazon Ish (39:15 DH uv'Sefer): The Shi'ur of Amah is a tradition from Moshe from Sinai based on the thumb (six Tefachim is 24 thumbs). In each generation, it depends on the size of an average thumb. The Gemara and Rashi in Menachos connotes that the Shi'ur is from the elbow to the end of the middle finger. We must say that this is for an average person, but really it depends on thumbs. Acharonim say that for us, an Amah is five Tefachim of thumbs. We rely on this, but where a six-Tefachim Amah is required, we add a Tefach.

Shulchan Aruch (396:1): A person may go four Amos anywhere, even if he left the Techum. We measure based on his Amah. If his Amah is small, he gets four Amos of an average person, each of which is six Tefachim.

Rema: These are spacious Amos.

Mishnah Berurah (5): This is whether he gets his own Amos, or average Amos.

Kaf ha'Chayim (5): The Mechaber disagrees. The Rema should have said 'some say...' He did not, for the words of the Mechaber could be understood like this.

Kaf ha'Chayim (6): We are lenient to allow spacious Amos for Techumim, which is totally mid'Rabanan.

Kaf ha'Chayim (7): A spacious Amah is half a finger bigger than a precise Amah.