Taken from Halachically speaking:
Pizza: How Many Slices is a Seuda?
When eating pizza one is faced with the question of
how many slices would constitute a shiur kevias
seuda to require washing and bentching?
Many poskim say that even one slice of pizza is a
meal for which one is required to wash and
bentch. 76 Others say the shiur is two slices, 77 while
others say that two slices would only constitute a meal if they are eaten with something else, such as a
knish. 78 As mentioned above, there is a dispute if
pizza is a meal food or a snack food. Therefore, one
should decide why he is eating the pizza: if it is for
a snack, then he should wash and bentch if he plans
on eating more than two slices, and if he is eating it
as a meal then he should wash even on one slice. 79
have to wash and bentch according to everyone. On buying a
slice of pizza and not a whole pie in regard to being a shaleim
see Avnei Yushfei 3:21:8.
page 24, Chai Moshe page 283, see Bais Avi 5:7-12.