The source of this video is https://www.kolhalashon.com/new/Media/PlayShiur.aspx?FileName=6033-20181017-111229-Rav_Yisroel_Yaakov_Sellam-The_Machlokes_Rav_Chaim_Noeh_and_the_Chazzon_Ish.mp3&English=True&Video=True&ShiurNum=161
Comment by Shlomo Moshe Scheinman:
Rav Sellam left out in the video some key points.
The Talmud teaches us (indirectly) that the size of the smallest mikva of water that is still kosher to be used as a mikva is 40 Seah which is equal to the amount of water displaced by 5760 eggs. The Talmud also tells us the size of the smallest mikva of water that is still kosher for various purity issues is 1 cubit long 1 cubit wide and 3 cubits tall.
Since the Talmud tells us how many halachic thumbs are in a cubit. If we knew the size of a halachic egg we could deduce the size of a halachic thumb.
According to the Chazon Ish, there is either a discrepancy between the size of Talmudic eggs and modern eggs or there is a discrepancy between the size of Talmudic thumbs and modern thumbs.
He resolves the problem by saying our modern eggs are smaller than Talmudic eggs. Rabbi (Avraham) Chaim Naeh disagrees. He says our modern eggs are not smaller than Talmudic eggs.
He also holds that modern thumbs when measured properly are similar in size to Talmudic thumbs.
Many commandments need a minimum volume or size. The calculations of the Chazon Ish change the minimum volume or size.
Rabbi Avraham Kook, Chief Rabbi of British Mandate Palestine used a combination of archaeology of the size of eggs found in ancient Egyptian pyramids as well as new manuscripts of the Gaonim to prove our modern eggs are not smaller than Talmudic eggs.
Rabbi Kook’s viewpoint is found in
הערות הרב קוק לאוצר הגאונים, לוין 5692 ביצה (יום טוב), 61.
I will also add that a relative of the Chafetz Chaim claimed that the Chafetz Chaim made regular use of a Kiddush cup that fit the smaller Rabbi Chaim Naeh measurement and not the Chazon Ish measurement.
A good supplement to Rav Yisroel Yaakov Sellam’s lecture is the audio lecture by Rabbi Chaim Eisenstein, Measurements in Halacha#1: The Size of a Reviis
At about 27 minutes 45 seconds of this audio lecture, Rabbi Eisenstein informs us that the grandson of the Chafetz Chaim reports that his grandfather, the Chafetz Chaim used a Kiddush cup that fits the measurements of Rabbi Chaim Naeh and does not fit the measurements of the Chazon Ish. Rabbi Eisenstein explains in the lecture why the Chafetz Chaim’s Kiddush cup does not contradict what he wrote in Mishna Brura.
In addition see https://www1.biu.ac.il/indexE.php?id=46&pt=20&pid=4&level=1&cPath=4&type=1&news=3318
This finding is an important contribution not only to the reconstruction of the ancient food basket, but also to the world of Jewish law (halakhah). The reconstruction of the egg from the City of David, as well as a comparison with later finds from the Roman period, indicate that the size of the chicken eggs and their volume were similar to the average size of an egg nowadays. The measure known as “kabeytza” [the size of an egg] has halakhic implications in various areas in measurements used by the Torah.
This blog post is located at https://vilnagaon.org/halachic-egg/