The lowest shiur is eighteen grams.
Explanation:
Yoma 80a:The Chachamim estimated that a person’s throat can’t hold more than the size of a chicken’s egg.
The Gemara in Kritut says: The Chachamim estimated that a person’s throat can’t hold more than two olives.
This Gemara seems to say the ta Zayit is half a Beitzah.
The Gemara in Eiruvin 80a writes: The size of two meals for an Eiruv is 18 Grogrot (figs). And on Daf 82b : R’ Shimon says two handbreadths are (two thirds) of loaf of bread when they equal three loaves per Kav.
This means that the amount for two melas is two thirds of a loaf when three loaves equal a Kav (2/9 of a Kav). A Kav has 24 Beitziim in it. So, the amount for two seudot is 2/9 of 24 which is 5 and a third eggs. Therefore, it comes out that five an third eggs equals 18 Grogrot which means that one Grogrot is less than a third of an egg.
A Kazayit is less than a Grogrot as it says in Shabbos 91a that Hotzaah requires a Grogrot which is more than a Zayit.
It seems that one Gemara says a Kazayit is half an Beitzah and one says it a third.
Rabbeinu Tam (Yom 80a) answers a Zayit is les tan third of an egg, The Gemara which says the that can’t hold more than two olives is because of its shape, that it’s round and therefore hard to put two in the throat at once.
The Ri answers A Zayit is half an egg, the Gemara in Eiruvin that two meals is 18 Grogrot is according to R’ Shimon but the Tannaim in the Mishna there who argue with hm understood that zayit is bigger (a half an egg).
The Rambam (Shabbat 8:5) writes a Grogrot is one third of an egg.
The Rambam rules like the Gemara in Eiruvin that a meal is eighteen Grogrot. The Pri Chadash (486:1) explains that the Rambam understood two zeitim and not more, it means Zayit is a regular shiur but it really means a Kotevet and one zayit that it doesn’t hold more than four zeitim.
The Rf and Rosh both agree with the Rambam and Rabbienu Tam that zayit is third of a beitzah.
The Shulchan Aruch is contradictory. In OH 486 write that a zayit is half a Beitzah (by Matzah) . In Hilchot Eiruvin 368:3: How much is an Eiruv, when there were eighteen people or less, the chachamim said one Grogrot for each one. But if there are more people, we only require eighteen Grogrot which are six eggs and some say eight eggs.
R Ovadia Yosef (Chazon Ovadia Shut 2:28) quotes the words of the Zera Emet that since The Rambam Rif and Rosh al hold that a zayit is third of a beitzah that is the actual halacha just by Matzah which’s from the Torah, he is stringent to require half a Beitzah.
How much is a Beitzah?
The Rambam in Perush Hamishnayot (Ediyot 1:2) writes that Revii’t is seven Derham.
The Rif sys that a Revi’it is like a Beitzah and half. Therefore, a Beitzah is eighteen Derham.
How much is a Derham?
Rabi Chaim Noeh measured that a Derham is 3 and fifth grams. Rabbi Hadar Margolin measured it recently that a Derham is only 3 grams.
So, a shiur of a Beitzah is 54 grams and zayit is (one third) 18 grams. However, of the mitzvah of Matzah the Shulchan Aruch is stringent for half an egg which is 27grams (according to R’ Chaim Noeh 30.6 grams.)
So ruled R; Ovadia Yosef that one hold be stringent like the Shulchan Aruch and eat half an egg an older person or a sick person can be lenient and eat a third of an egg but without saying Al Achilat Matzah.
