Mishnah/Seder Zeraim/Tractate Berakhot/Chapter 7/2
Women, slaves, and minors, Zimmun is not said for them.
How much [must one eat] to say Zimmun? Like an olive. Rabbi Judah says, Like an egg.
Like an olive-- To say Zimmun, one must have eaten food at least the volume of an olive, or in the opinion of R. Judah, an egg.