Gemara

(Mishnah): If the Kometz was Huktar in two stages, it is Kosher.

(R. Yehoshua ben Levi): It is Kosher in two stages, but not in more than this;

(R. Yochanan): It is Kosher in two stages, or even in more than two.

(R. Zeira): R. Yehoshua ben Levi holds that a Kometz must be at least two k'Zeisim, and Haktarah must be at least a k'Zayis (since it is called 'Achilah'). R. Yochanan holds that a Kometz can be less than two k'Zeisim, therefore Haktarah can be less than a k'Zayis (since we know that Haktaras Kometz can be in two stages).

Shabbos 92a (R. Elazar): If one was Motzi something more than 10 Tefachim above the ground he is liable, for this is how Bnei Kehas carried.

(Rav): The Leviyim were 10 Amos tall, and the Mizbe'ach was 10 Amos. When something is carried on poles, a third is above (the poles), and two thirds are below.

Alternatively, we learn from the Aron (even if the Leviyim were our height, i.e. 18 Tefachim up to the shoulder, less than eight Tefachim hung below).

Yoma 80a (R. Elazar): One who eats Chelev nowadays must write down the Shi'ur. Perhaps a later Beis Din will increase the Shi'ur.

(R. Yochanan): The Shi'urim for punishments are a tradition from Sinai.

(Beraisa - Others): The Beis Din of Ya'avetz enacted Shi'urim.

Correction: The Shi'urim were forgotten, and they re-established them.

Eruvin 83a: Rebbi measured and found that k'Beitzah (regarding Tum'ah) is an egg and excess, i.e. another one part in 20 for each egg.

Rishonim

Tashbatz (3:33): It is best to measure for a Mikveh based on fingers of average sized people. Eggs vary more from place to place. When we gauge the size based on eggs, it is much smaller than based on fingers. If there are two different Shi'urim, it is proper to be stringent about Torah laws.

Ramban (Bereishis 6:19 DH v'Im): Do not say that the ark was 300 Amos of No'ach, and he was very big (to explain how all the animals fit in the ark). If so, also the people and animals were bigger before the flood! Also, the Amos are Amos of the Torah.

Mishneh Halachos (4:118): Tashbatz did not want to say that the Shi'ur changes in every generation. Also the Ramban says that the Shi'urim of the Torah do not change. In Yoma 80a, we conclude that Shi'urim are a tradition from Sinai, i.e. and they do not change! It is known that people are getting smaller. Nidah 24b discusses the tallest man of eight consecutive Doros (generations), and each reached only the shoulder of the previous Dor. Sizes vary with place; it is known that Russians are shorter than Americans.

Rivash (295): Do not say that nowadays people are smaller due to our sins. We rely on the measures of earlier Doros. Mishnayos Mikva'os (which give the Shi'ur for Mikva'os) were taught nowadays (i.e. after the Churban), just like the Mishnah (Bava Basra 100b) which gives dimensions of burial caves (which are appropriate for people nowadays).

Poskim

Shulchan Aruch (OC 486:1): Some say that the Shi'ur of a k'Zayis is about half a Beitzah.

Mishnah Berurah (1): Some Acharonim proved that the eggs found nowadays are much smaller, about half the size that they were in the early days when Chachamim estimated (Shi'urim). Based on this, whenever the Shi'ur is half a Beitzah, one should gauge this with a whole egg nowadays. Sha'arei Teshuvah says that for a Torah Mitzvah (such as Matzah), one must eat k'Beitzah, and for a Mitzvah mid'Rabanan such as Maror, Birkas ha'Mazon, the Afikoman and the four cups of wine, a third of an egg suffices.

Tzlach (Pesachim 116b DH v'Ho'il): The eggs found nowadays are about half the size that they were in the early days when Chachamim estimated (Shi'urim). The Shi'ur for Chalah is 43.2 eggs. The Tosfos Yom Tov measured that this is the old pint of Prague. I found that it is less. I made a Kli that holds the Shi'ur for Chalah (YD 324, based on thumbs). It holds two pints! This is a contradiction in the Shi'ur for Chalah. This Shi'ur and for Mikveh are both traditions from Sinai. Either thumbs became bigger than they were in the days of the Tana'im, or eggs became smaller. We know that the Doros are getting smaller. Our thumbs are not bigger than those in the days of the Gemara, so we must say that the eggs are smaller. A k'Zayis is like an egg nowadays. One should use this for Matzah, Maror and all Shi'urei Torah.

Chasam Sofer (1:127 DH Tu): Our egg is like a k'Zayis of their time. How did Hillel eat Pesach, Matzah and Maror, and swallow (a k'Zayis of each of) them, i.e. three of our eggs, at once? No one can swallow this much, even if it is mashed!

Bi'ur Halachah (271 DH Shel): The Tzlach's question is strong, but there is also a question against him. It is clear from Yoma 80a that Malei Lugmav from both sides (the amount that one can hold in his mouth when both cheeks stick out) is more than a Revi'is in a standard person. Tosfos and the Ran say so. If a Revi'is is three of our eggs, Malei Lugmav of an average person should exceed this! The Tzlach proved that fingers are the same, and presumably the same applies to all limbs. I tested Malei Lugmav of many average men, and the biggest was two eggs with the shell. Our eggs did not get much smaller! (Or, perhaps Malei Lugmav from one side is a Beitzah, and Malei Lugmav from both sides is two eggs.) For Torah laws, e.g. Matzah on Pesach night and Kidush on Shabbos night, one should be stringent like the Tzlach and Chasam Sofer. One may rely on the custom (to consider our eggs to be a Beitzah) for morning Kidush and the cup over which we say Birkas ha'Mazon. It should hold at least two eggs with the shell (based on my measurement of Malei Lugmav from both sides). Even the Tzlach only requires that the cup hold three of our eggs, but he agrees that it suffices to drink one's own Malei Lugmav, even if it is only a third of the cup. Derech ha'Chayim says so. Some disagreed, but we need not adopt two stringencies.

Bi'ur Halachah (16:1 DH l'Shuk): If a Talis Katan is an Amah by an Amah, surely one may bless on it. The Amah is six Tefachim; one need not use our Amos.

Mishneh Halachos (8:194): The Mishneh l'Melech (Hilchos Ma'aseh ha'Korbanos 14:14) cites a Yerushalmi which says that if a Kohen's hand does not hold two k'Zeisim, he is Pasul for Avodah. Nowadays, almost no one's hand holds two eggs of flour! If people did not get smaller, and our eggs are like a k'Zayis, almost no Kohen was Kosher for Avodah! Tashbatz and Ba'al ha'Itur already knew that the measurement based on eggs is smaller, yet they were not stringent to use the Shi'ur based on thumbs. The Tzlach says that the custom is to use a cup that holds a Beitzah and a half for Kidush. The Gra and all Chachamim until about 100 years ago relied on the smaller Shi'ur. The Shi'urim of k'Zayis, k'Beitzah and Revi'is arise every day regarding Berachos and Netilas Yadayim. It disgraces early Chachamim to say that they were not Yotzei. It is difficult to say that nature has changed. I say that the measurement is based on the bone of the thumb. We eat well, so our thumbs are fatter.

Note: Above, he proved from Nidah 24b that the Doros became smaller!

Chazon Ish (Mo'ed 39:1): Shi'urim are based on one's estimation, just like one estimates to take Terumah. The tradition from Sinai for Shi'urim was based on man's estimation. Chachamim fixed the Shi'ur for Yisrael, e.g. to say that 'k'Zayis' refers to Aguri (a certain species of olive), barley refers to what grows in the Midbar, etc. If Chachamim did not fix the Shi'ur, or if what was fixed is not found, again everyone uses his own estimation.

Chazon Ish (4): If a Shi'ur was said in length, it was given for Chachamim to fix and individuals to estimate only in length. Likewise, a Shi'ur of weight may be fixed and estimated only in weight. It follows that the Shi'ur changes with nature. If Chachamim said what is the weight of the Shi'ur of flour or a Revi'is, this is not a Shi'ur Torah.

Chazon Ish (5,6): Chachamim of the Gemara, who were a Sanhedrin for Klal Yisrael, gave us two ways to know volume, through eggs or thumbs. One who knows to estimate based on one of them need not seek the other method. We are not concerned if the eggs got bigger or smaller. However, if one knows that there is a discrepancy, he should adopt the measurement based on thumbs. It is the basis of the Tefach and Amah, which the Torah mentions. Further, the thumb corresponds to Chazal's Midos, and the eggs changed. Rebbi enacted to add to the Shi'ur of eggs, for he knew that they got smaller. Tosfos (83a DH Yeserah) says that Midbar eggs were bigger. Rebbi did not fix the Shi'ur based on thumbs, for it is harder to gauge exactly an average thumb. Alternatively, as long as we still have a measure of earlier Chachamim, we rely on it. We rely on thumbs only when the other Shi'ur (of eggs) is not available. As long as we had coins of Moshe, we relied on them for the Perutah, Dinar and Shekel. After they were lost, we rely on the Ge'onim's tradition regarding the weight of barley. As long as we had the Amah of Moshe, we were not concerned for the size of thumbs. The Noda bi'Yehudah and Gra found that the measures based on thumbs and eggs are different. It is unreasonable that our thumbs increased, so they concluded that the eggs decreased, so they established that Yisrael measure based on thumbs. The early Doros did not err. Since they did not notice a discrepancy, they were entitled to rely on the measurement based on eggs. The Noda bi'Yehudah, Gra, Chasam Sofer and R. I. Z. Margaliyus fixed that we follow the measurement based on thumbs. It turns out that an Amah is 58 centimeters. We rely on the Shi'ur based on thumbs, even if the size of thumbs will change.

Chazon Ish (6 DH Achar): I found a Gaon (Eruvin 83) who says that the Shi'ur Chalah was given in eggs. On Sinai, the Torah authorized Chachamim to fix Shi'urim in eggs and Peros, for these are found everywhere. Hash-m knew that Yisrael would be exiled and lose the weights and measures of the days of Moshe. The Shi'ur of eggs and Peros depends on the observer (to estimate what is average). Since eggs and Peros were given to facilitate estimating the Shi'ur, surely they are based on the current time and place.

Ohr l'Tziyon (3, Introduction, Shi'urei Mitzvos 2): The simple Perush of the Gaon is that the Torah authorized Chachamim to fix Shi'urim in eggs and Peros, for these would never change! Sefardim follow our tradition, that the measures did not change. However, some Sefardi Gedolim were stringent for Torah Mitzvos such as Mikveh, the four Minim and Sukah.