The poskim differ regarding the shiur of k’zayis in relation to an egg. The Shulchan Aruch states that a k’zayis is the volume of half an egg. The Rambam held it is only a third of an egg. For a mitzva d’oraisa like Matza, you should be machmir to eat at least a half of an egg’s worth of matza. (MB1)
Never eat only a third of an egg’s worth of food — you are putting yourself into a sofek berachos. Eat either less than a third of an egg’s amount, or a half or more which is a kezayis according to all opinions. (MB1)
There are Achronim who prove that a kezayis nowadays is actually the size of a modern day egg. The Shaaray Teshuva says to be machmir for this for mitzvos d’oraisa. What emerges is that the mitzva to eat the first kezayis of matza amounts to a full egg’s worth. (MB1) # SECTION III — DAAVENING FROM PESACH UNTIL SHAVUOS
