If I usually make the beracha of mezonos on 1 slice of pizza and hamotzi on 2 slices,and I decided to eat 2 slices for a meal so I started out saying hamotzi and then felt abnormally full. Do I say birkas hamazon?
Even if one eats less than the usual quantity that constitutes a full seuda on its own (meal), being full is itself a reason for reciting birkas hamazon, provided one eats a quantity of mezonos that would be considered a meal when eaten together with side dishes. Provided one becomes full, one (store-sized) slice of pizza would therefore be sufficient.
Sources: See Mishnah Berurah 168:24; Vezos Haberacha p. 29.