Personally, I prefer "two slices of pizza
is enough for me," because your are talking about the combined amount, not the individual slices. If you were talking about the individual slices you would use a plural verb: "Two slices of pizza are bigger than the others." (
Each of the two slices is bigger than the others; however, you could not say that
each of the two slices is
enough for me, they are only 'enough" when considered together.)