Although the mitzva d'oraita is to thank God only when you are satiated from the meal, chazal decreed that we say birkat ha'mazon even when eating only the volume of a k'zayit and even if not satiated. So you must still say birkat ha'mazon after eating at least 1.3 fl. oz. (39 ml, or 1/6 cup) of bread within four minutes--even if you are not full.  This is known as birkat ha'mazon d'rabanan.

K'zayit is variously defined as being the volume of 1/3, ½, or 1 whole egg; 0.6 - 1.9 fl. oz. (18-56 ml), depending on the application.

• K'zayit for Seder Matza and Afikoman:  1.9 fl. oz. (56 ml) of matza.
• K'zayit for Bracha Achrona and Birkat HaMazon: 1.3 fl. oz. (39 ml, or 1/6 cup) of solid food.

Beitza, K'Beitza = 1.9 fl. oz. (56 ml); an egg size by definition.