שיעור כזית

How big is a כזית in modern day measurements?

The shiur given is for matzah. (1/6 machine matzah).

What about for an egg? Half? Third?

כזית is a measurement of volume and it applies to matzah or eggs or anything else.

I understand. A conversion to another food item would be helpful. Thank you

There is no way real way to do that in a practical way. Understand the volume of a kezayit and then try applying it yourself to other products.