Gemara

(Mishnah - R. Yehudah): Old (well-fermented) Se'or is put into the measuring Kli, and flour is added to complete the Isaron.

Chachamim: If so, there will be too much or too little flour!

53b (Rav Chisda): If the Se'or was kneaded thickly (with little water; it has more flour than the same volume of pure flour), there will be too much flour in the Minchah. If it was kneaded thinly (with much water), there will be too little flour.

Question: In any case, the measure was filled to an Isaron!

Answer (Rabah and Rav Yosef): The original volume of the flour (i.e. when it was flour) must be an Isaron. Since the Se'or helped fill the measure, the Isaron lacks (or has an excess of) the difference in volume of the Se'or and its flour.

87a (Beraisa - R. Meir): There were two one-Isaron measures in the Mikdash. One of them held an Isaron Machuk (the overflow is wiped away). The other held an Isaron Gadush (heaping).

The Isaron Gadush was used for all Menachos, except for Chavitim, for which the Machuk was used.

90a (Mishnah): All the measures in the Mikdash were Gedushos, except for those of (Chavitei) Kohen Gadol, which were Mechukos.

(Rav Chisda): This means that all the measurings were Gedushos.

Pesachim 48a (Beraisa #1 - R. Yishmael): On Pesach, one may make a dough up to two Kavim of wheat, or three Kavim of barley.

Answer #2 (R. Noson citing R. Eliezer): One may make up to three Kavim of wheat, or two Kavim of barley.

Contradiction (Beraisa #2 - R. Yishmael): One may make up to three Kavim of wheat, or four Kavim of barley.

Resolution: Beraisa #1 discusses good grain. Beraisa #2 discusses poor grain.

(Rav): One may knead up to a Kav of Melugna'ah (a certain place) on Pesach; this is also the Shi'ur from which Chalah must be taken.

Question (Mishnah): Chalah must be taken from a dough of slightly more than five Lugim (i.e. quarter Kavim, also called Reva'im) of flour.

Answer: Rav teaches that this equals one Kav of Melugna'ah.

(Rav Yosef): Nowadays, women do not bake more than three Lugim at a time.

Objection (Abaye): You intend to be stringent. This causes a leniency. It exempts the dough from Chalah!

Answer (Rav Yosef): They do like R. Eliezer;

(Mishnah - R. Eliezer): If one removed loaves (each is less than the Shi'ur to obligate separating Chalah) from an oven and put them in a basket, the basket makes them join (to the Shi'ur).

(Rav Yehudah): The Halachah follows R. Eliezer.

Question: R. Yehoshua ben Levi taught that this is only regarding (wide round) loaves of Bavel that stick to each other in the oven, but not to long thin loaves that do not!

Answer: R. Chanina taught that it applies even to long thin loaves. (Rav Yosef holds like R. Chanina).

Rishonim

Rambam (Hilchos Bikurim 6:15): The Shi'ur Chalah is 43.2 eggs. This is the weight of 68 and two thirds Sela'im of wheat flour of Mitzrayim.

Rambam (Hilchos Chametz 5:12): If one kneaded a dough (for Pesach) larger than the Shi'ur for Chalah, it is permitted. The Shi'ur is 43.2 medium eggs, not like their weight.

Rosh (Pesachim 3:6): More than five Reva'im of flour obligates taking flour. R. Yonah says that if one kneaded more than this, Chachamei Tzarfas forbid during Pesach. I permit. This Shi'ur is the most dough that one can work with using his hands. As long as one works with dough, it does not become Chametz. Working with a big dough is no worse than not working at all with a small dough. We do not delay the Shi'ur to walk a Mil (before the Matzah is baked), therefore, it is permitted. However, Ge'onim said that l'Chatchilah one should not leave dough for a moment without working it. Therefore, l'Chatchilah one should not knead more than this Shi'ur. How could someone think to forbid Matzah baked from more than a Shi'ur? Each loaf of Lechem ha'Panim was from two Esronim (94a)!

Note: One could say that we are less zealous than Kohanim, and our ovens are not as hot as theirs!

Rosh (Hilchos Chalah 1:2): It seems that we measure the five Reva'im of Chalah with grain, and not with flour, for in the Midbar they collected an Omer of manna, and afterwards ground it. In Ashkenaz, women use a heaping measure of flour. I assumed that the volume of the flour is more than the volume of the grain. The Mishnah (Chalah 1:4) connotes like this, but it is not a proof. Mishnah 2:6 says that the Shi'ur is five Reva'im of flour. This shows that the Shi'ur is of flour.

Note: In Pesachim 48, the Shi'ur of flour (one and a quarter Reva'im) is less than the Shi'ur of grain (all say that it is at least two Kabim)! Perhaps the Rosh thought that nowadays the grain is much better, so it produces more flour).

Question (Teshuvas ha'Rosh 2:15): All my Rebbeyim measured the 43.2 eggs of the Shi'ur Chalah with flour that fills the measure evenly. We learn from the Midbar. They collected an Omer of manna, and afterwards ground it. Presumably it increased a lot! Also we should require flour ground from five Reva'im of grain, and five Reva'im of flour. The Mishnah in Chalah refers to flour that comes from five Reva'im of grain. It is better to use a heaping measure, for there is a Safek Berachah l'Vatalah (if there is not enough flour).

Answer (Teshuvas ha'Rashba 1:465): According to this premise, we could never know the Shi'ur. Who knows how much the manna increased (through grinding)?! Surely, it was better than (increased more) than any other species. Wheat and barley increase different amounts, and good and bad grain increase different amounts within one species. If we are stringent due to Safek Berachah l'Vatalah, this will cause a leniency. If we exempt five Reva'im of flour, people will eat bread that is Tevel to Chalah! (This means that one was obligated to separate Chalah from it, but he did not.) Also, people will knead a big dough (during Pesach)! Rather, the Shi'ur is five Reva'im of flour, and not flour that comes from five Reva'im of grain.

Note: Since the Rosh considered this a Safek, presumably he intended to be stringent for both Shi'urim.

Tosfos (52b DH Chaserah): (Since it is a problem if the Se'or is more or less dense than flour,) this shows that when measuring the Shi'ur for Chalah or Pesach, one may not press on the flour.

Mordechai (Pesachim 598): The Riva says that we do not use a heaping measure for Pesach. Since the Shi'ur is an Isaron, we learn from Chavitei Kohen Gadol, which were an Isaron, and the Godesh was within the Midah. All other Midos in the Mikdash were overflowing, like it says in Menachos.

Poskim

Shulchan Aruch (OC 456:1): On Pesach we do not knead a dough larger than the Shi'ur of Chalah, which is 43.2 medium eggs. One fills a Kli with water, pours some out, puts in 43 eggs, and fills the Kli with some of the water he poured off. A Kli that holds the remaining water holds 43 eggs.

Beis Yosef (DH v'Chasav Rashi): R. Yerucham says that to measure 43 eggs, we puncture a medium egg, remove the contents, and fill it with water 43 times. A Kli that holds the water holds the correct measure. He also wrote like the Tur says (like the Shulchan Aruch, to fill a Kli...). Either way is about the same.

Darchei Moshe (1): It is astounding to say that the shell does not change the measure! Perhaps he holds that we put 43 peeled eggs into the Kli. The Poskim connote unlike this.

Note: The shell is about a quarter of the volume of the egg (Shach YD 86:15, Magen Avraham 210:2 from the Shlah; Chasam Sofer (Chulin 88a DH Aba) citing Seder Hagadah of Maharil). One cannot call this 'about the same'!

Bach (1): The Rambam holds that we measure with eggs themselves, and not with their weight. The Tur explains that we measure the eggs themselves, unlike the Perush in R. Yerucham that we puncture it and fill it with water, for then the Shi'ur is lacking the volume of 43 egg shells.

Kaf ha'Chayim (1): One may not make a big dough even if he intends to immediately split it into two. One may not knead small doughs and combine them, lest it become Chametz while forming the loaves.

Kaf ha'Chayim (3): Nowadays that a woman makes a big dough and gives to others to Lakatef (smear it), one may make a big dough.

Kaf ha'Chayim (2): One opinion says that our ovens are bigger, and it is better to make a big dough. All others disagree, but we cannot protest against one who relies on the first opinion. One should be stringent even if many help him.

Shulchan Aruch (ibid.): After filling the Kli with flour, he must scrape the excess off the Kli. It may not be heaping.

Beis Yosef (DH Kasav Rabeinu): In YD 324, the Tur wrote that we measure five Reva'im of grain, and not of flour, even though this is more than this Shi'ur of flour. Therefore, when one fills the measure with flour, it must be heaping. He learned from the Rosh. However, the Rosh was unsure. Regarding Pesach perhaps the measure need not be heaping, therefore, the Tur did not mention this; he is stringent. The Rashba holds that also for Chalah the measure is not heaping. In Hilchos Chalah, the Rosh wrote that women use a heaping measure, and gave a reason for this, but his final words lean to measure with flour even for Chalah, and all the more so for Pesach. The Mordechai brought a proof from Chavitei Kohen Gadol that the measure is Gadosh. This is not a proof, for perhaps we learn from the other Midos, which were heaping! In any case, also he holds that we do not use a heaping measure. We hold like this.

Bach (DH Ela): The Kohen Gadol's measure contained an Isaron without a Godesh. The other measures contained an Isaron with a Godesh. The Beis Yosef erred about this, therefore, he said that this is not a proof. However, Rashi and Tosfos explain that the Kohen Gadol's measure was different lest the flour spill when he divides it in half! Perhaps we learn from other measures, and use a Kli that has an Isaron with the Godesh! The Riva explained that the Kohen Gadol's measure was not Gadush, for if so he could not divide it evenly in half. We learn from here that we do not use a Gadush measure for Pesach, lest it be more than an Isaron. One may bless on (taking Chalah from) an even measure. All Poskim say so except for the Rosh, who was unsure; and it seems that he retracted at the end.

Gra (DH Lemal'oso): Menachos 53b proves that the volume of the flour (not the grain) must be an Isaron.

Rema: One may not force the flour into the measuring Kli, for then it will not knead well.

Beis Yosef (DH v'Zeh): Orchos Chayim says we may excommunicate people who use a heaping measure for Pesach. Kol Bo was unsure whether we measure with soft flour, or compressed flour.

Gra (DH v'Lo): Tosfos says that one may not press on the flour, for then too much fits in the measuring Kli. Maharil says that this causes that water not mix with the flour in some places, and (after baking, if it gets wet,) that flour can become Chametz.

Mishnah Berurah (4): L'Chatchilah, one should not even put his hand on the flour.

Rema: When putting the flour into the Kli, it is good to say that he does so l'Shem Mitzvah.

Kaf ha'Chayim (20): It is good to say so also when kneading, making the Matzos, drawing the water and putting it on the flour.