Gemara

82b (Mishnah - R. Meir): The Shi'ur of two meals [for Eruv Techumim] is the amount [of bread] one eats in two weekday meals;

(R. Yehudah): It is the amount of two Shabbos meals;

Both of them intend to be lenient.

Answer #3 (R. Yochanan ben Brokah): It is a loaf one buys for a Pundiyon (one part in 48 of a Sela) when wheat sells for four Sa'im for a Sela;

(R. Shimon): It is two thirds of a loaf that is a third of a Kav.

83a (Beraisa): "Reishis Arisoseichem" is the amount of dough that people baked in the Midbar.

One received "Asiris ha'Eifah" of manna each day. Therefore, one must take Chalah from a dough with seven Lugim and excess of flour (the volume of 43 and a fifth eggs in all).

One who eats this amount [each day] is healthy and blessed. One who eats more than this is gluttonous. If one eats less, his innards are spoiled.

Berachos 42a: Rav Huna ate 13 loaves. Each was a third of a Kav. He did not bless afterwards.

(Rav Nachman): He ate these due to hunger. Since others fix a meal on such an amount, he must bless on them!

(Rav Yehudah citing Shmuel): One blesses ha'Motzi on Pas ha'Ba b'Kisnin (baked Mezonos that people do not normally fix meals on) only if he fixed his meal on it.

Kesuvos 64b (Mishnah): One who feeds his wife through an intermediary must supply every week at least two Kabim of wheat... a half Kav of Kitnis (a legume), half a Log of oil, and a Kav of dried figs.

This applies to the poorest man in Yisrael. Each man must increase according to his standing.

Rishonim

Rif (Berachos 30a): We conclude that if one ate an amount [of Kisnin] that others do not fix a meal on, like Kisnin (without fixing on it), he blesses Borei Minei Mezonos beforehand and nothing afterwards. If he fixed his meal on it, he blesses ha'Motzi and Birkas ha'Mazon, like Shmuel taught.

R. Yonah (DH Amar): Rav Nachman says that if others fix on this amount, even one who did not fix on it must bless. If others do not fix on this amount, even one who fixed on it does not bless. We follow the majority. We are not concerned if he fixed on it. Batlah Daito Etzel Kol Adam (the Halachah is based on average people).

Rambam (Hilchos Berachos 3:9): If a dough was kneaded with honey, oil or milk, or spices were mixed in and he baked it, it is called Pas ha'Ba b'Kisnin. Even though it is bread, he blesses Borei Minei Mezonos, or ha'Motzi if he fixed his meal on it.

Rosh (Berachos 6:16): The Yerushalmi says that if one does not bless Birkas ha'Mazon, one does not bless ha'Motzi beforehand. This refers to species other than bread, e.g. Pas ha'Ba b'Kisnin if he was not satiated from it.

Rosh (30): Rabbeinu Moshe says that if others do not fix on this amount, even one who fixed on it does not bless. We follow the majority. Even if he fixed on it, Batlah Daito Etzel Kol Adam. The Ra'avad says that if he fixed on it, he blesses ha'Motzi and Birkas ha'Mazon even for Mashehu. Rabbeinu Moshe's opinion is better. If he intended to eat a little, and in the end ate a Shi'ur that others fix on, he blesses Birkas ha'Mazon, even though initially he did not bless ha'Motzi. If he ate it like Kisnin, he blesses Borei Minei Mezonos and me'Ein Shalosh. If he fixed on it, he blesses ha'Motzi and Birkas ha'Mazon, like Rav Yehudah taught.

Tosfos Rid (Kesuvos 64b DH Masnisin): How will two Kavim suffice a woman for a week? Everyone ate an Omer (43 and a fifth eggs) per day. This is almost two Kavim (48 eggs)! Rav Shalom Gaon answered that for feeding a wife, Chachamim gave a minimal Shi'ur to keep a person alive. This is the Shi'ur for an Oni. A more honored person gives according to his honor, up to an Omer per day.

Note: R. Yonah (in Shitah Mekubetzes 64b DH v'Hikshu) says that since he gives accompaniment (Kitnis...), she needs less wheat. Also, women eat less than men, for they work less. This is a Chidush. Even with the accompaniments, she gets much less an Omer per day. Also women ate an Omer each day in the Midbar! Tosfos Rid's question assumes that the manna satiated no less than wheat. Perhaps this is a Kal va'Chomer, for there was no waste in it. He must explain that the hunger in the Midbar ("va'Ya'ancha v'Yar'ivecha" - Devarim 8:3) was psychological, due to inability to save food for tomorrow. Alternatively, Hash-m caused people to feel hunger, so they would appreciate the manna more. Mishnas R. Aharon (1 p.9) connotes like this.

Ya'avetz (83b DH Gemara): Chachamim estimated for an average person of their generation.

Poskim

Shulchan Aruch (OC 168:6): One blesses Borei Minei Mezonos on Pas ha'Ba b'Kisnin, and me'Ein Shalosh afterwards. If he ate an amount that others fix on, even if he was not satiated, he blesses ha'Motzi and Birkas ha'Mazon.

Beis Yosef (DH Pas): Rashi, the Rif and Rosh explain that Rav Huna ate Pas ha'Ba b'Kisnin.

Beis Yosef (DH v'Chasav): We hold like the Rosh and R. Yonah, that one blesses ha'Motzi and Birkas ha'Mazon only for a Shi'ur that most people fix on.

Beis Yosef (DH veha'Agur): The Agur brings from R. Avigdor that we discuss fixing one's morning or evening meal, but not a haphazard meal. (People used to eat only two meals a day - PF.) This is from Shibolei ha'Leket (159). This is wrong. It is for fixing any meal. This is only if he ate a Shi'ur that people fix on, like I brought above.

Magen Avraham (13): If one fixed his meal on it, even if he ate with it meat and other things, and had he eaten only the Kisnin he would not be satiated, even so he blesses ha'Motzi and Birkas ha'Mazon. Tosfos (42a DH Lachmaniyos) connotes like this regarding rolls. He says "if one fixed his meal on it, e.g. on Purim..." Semak wrote "on the night of Purim, or the night [before] Bris Milah..." Presumably, on Purim one does not fix only on rolls. If one ate only Kisnin, he must eat as much as others eat of Kisnin alone to be satiated. If we would estimate based on the amount people eat with other things, this will vary! Eruvin 82b shows that one can estimate. This requires investigation. If he is satiated from it, and also others are normally satiated from it when they accompany it with something, even if he ate it alone, he blesses Birkas ha'Mazon, since he was satiated from it.

Birkei Yosef (6): The Magen Avraham's proofs are not decisive. Some ask that matters with different Shi'urim do not join (Me'ilah 17a). There is no Shi'ur for Kevi'us (fixing) on meat. I say that one can distinguish. In practice, this requires investigation.

Kaf ha'Chayim (47): The Shi'ur of Kisnin that one fixes on along with other foods is not uniform, for the more one eats of other foods, the less Kisnin he eats!

Birkei Yosef: One should eat less than 54 Drams (three eggs) of Pas ha'Ba b'Kisnin, which is not Kevi'us Seudah, or more than 72 Drams (four eggs), and bless Birkas ha'Mazon. If he ate 54 and cannot complete to 72, he blesses Birkas ha'Mazon.

Shiyurei Berachah (1): Rav Shalom Gaon and the Tosfos Rid hold that the minimum to live on is three eggs per meal, with two extra meals in the week. This supports that the Shi'ur that others fix on is more than three eggs. We do not discuss Aniyim. However, the Rambam (Hilchos Ishus 12:10) says "we apportion for a woman two average meals per day of people in that city." This shows that a half-loaf is an average meal. (Note: The Gra (EH 70:4) says that the Rambam explains that the Mishnah discusses Shi'urim for Eretz Yisrael. R. Yochanan ben Brokah says that a loaf is two meals.) Alfasi Zuta says that a half loaf is the Shi'ur for Kevi'us Seudah. However, Rashi and the Rambam argue about whether this is three or four eggs. Therefore, one should not eat between three and four.

Mishnah Berurah (24): The Acharonim say that if one ate just Kisnin, one must eat as much as others eat of Kisnin alone for satiation. If he ate with meat or other accompaniments, it suffices to eat as much as others eat of it for satiation with accompaniments. If he ate it alone and was satiated from it, even though others would be satiated from it only with accompaniments, we do not say Batlah Daito, and he blesses ha'Motzi and Birkas ha'Mazon. For Eruv Techumim, the Shi'ur of a meal is three or four eggs. Several Acharonim say the same here. The Gra and others say to bless ha'Motzi and Birkas ha'Mazon only for a fixed meal of morning or evening, like the Beis Yosef brought from Shibolei ha'Leket. The Eshkol agrees. The Shulchan Aruch connotes like this. L'Chatchilah it is good to be stringent not to eat four eggs, especially of something that is a [Safek bread,] Safek Pas ha'Ba b'Kisnin. We are lenient [to consider it Pas ha'Ba b'Kisnin], but surely one should be concerned for the stringent opinion.

Mishnah Berurah (25): One blesses also Al Netilas Yadayim, like for proper bread.

Kaf ha'Chayim (45): The Shulchan Aruch did not specify how much others fix on. It is difficult to say that this depends on what one sees others do! It seems that we learn from Eruv, i.e. three or four eggs according to the Rambam or Tur, respectively. The Shulchan Aruch rules Stam that it is three eggs in 368:3 and 409:7.

Kaf ha'Chayim (46): Beis David, Mateh Yehudah (or Mateh Yosef), the Chida and Pesach ha'Dvir say that we measure based on weight. An egg is 18 Dram. This is for all Mitzvos - Matzah, Maror, Berachah Acharonah, Esrog, Eruvei Chatzeros...

Shulchan Aruch (ibid.): If he intended to eat a little, and in the end ate a Shi'ur that others fix on, he blesses Birkas ha'Mazon, even though initially he did not bless ha'Motzi.

Gra (DH v'Im mi'Tchilah): It does not matter what he blessed beforehand. The Rosh (16) brings so from the Yerushalmi. It asked [against 'if one does not bless Birkas ha'Mazon, one does not bless ha'Motzi beforehand'], and answered [that this does not discuss quantity, rather, species].

Damesek Eliezer: The Yerushalmi teaches that for other species (not bread, rather, Kisnin), whenever one eats a Shi'ur for which one blesses afterwards Birkas ha'Mazon, he blesses beforehand ha'Motzi. Other species depend on the Shi'ur. The Berachos before and after are not Me'akev each other. If one did not intend to eat Shi'ur Kevi'us Seudah, but in the end he did, he blesses Birkas ha'Mazon even though he did not say ha'Motzi. (Likewise, if one intended to eat Shi'ur Kevi'us Seudah, but in the end he did not, he says me'Ein Shalosh, even though he said ha'Motzi.)

Shulchan Aruch (ibid.): If he ate a Shi'ur that others do not fix on, even if he fixed on it, he blesses only Borei Minei Mezonos and me'Ein Shalosh. His opinion is Batel to that of average people.

Mishnah Berurah (26): If he reconsidered, and plans to eat an additional amount of Kevi'us Seudah, he washes and blesses Al Netilas Yadayim, ha'Motzi and Birkas ha'Mazon.

Kaf ha'Chayim (51): Some hold that one who blessed Borei Minei Mezonos on bread was Yotzei. If so, he need not bless ha'Motzi after deciding to eat a Shi'ur. He washes and blesses Al Netilas Yadayim, without diverting his mind from the Borei Minei Mezonos that he said before. If he will eat between three and four eggs, he washes without a Berachah, since there are two opinions about this.

Kaf ha'Chayim (52): If one will fix on Kisnin, and there is also bread in front of him, he should bless ha'Motzi on the bread.

Bi'ur Halachah (DH Af): Surely, an old or young person whose nature is to eat a small amount, blesses ha'Motzi and Birkas ha'Mazon for an amount that he always fixes on, since all people like him do so.