48:3 The size of a normal meal’s portion is not based on an individual’s personal eating habits but on that of the majority, i.e., what most people would eat at lunch or dinner to be satisfied. Even if one is personally not satisfied, pas haba’ah b’kisnin would still be considered fully bread in an amount that would satisfy most people. (Bi’ur Halacha on 168:6, s.v. af al pi, says that children and the elderly, who typically eat less than most adults, must base their amounts on the average for their age group.) If one eats pas haba’ah b’kisnin with other foods, he must take the average person’s appetite into consideration. If he ate pas haba’ah b’kisnin without other foods and was satisfied with a small amount, even if others would only be satisfied if they ate it with other foods, he must still treat it like regular bread. (There issome discussion as to the actual sizes involved. Mishnah Brurah 168:24 suggests that the volume of four eggs seems to be about the size of an average person’s meal.)
48:4 If one intended only to eat a little pas haba’ah b’kisnin and said the bracha of mezonos on it, then he went on to eat a normal meal’s worth, what should he do? If the amount he wants to eat after changing his mind is only a meal’s volume when combined with his original amount, he should just eat it and bentch afterwards. If the amount he wants to eat is a meal’s worth in and of itself, he must wash and say hamotzi; he does not say al hamichya on what he already ate because it will be covered when he bentches.