For info, a pizza in BrE is never called a pie. It's just called a pizza. A pie is a completely different food.
The answer to the original question is the same. If I'm eating a couple of slices or even half the whole pizza, I'm eating "pizza". If I'm eating the entire thing, then I'm eating "a pizza". However, if I were talking in advance of my dinner plans, I would say "I'm having pizza for dinner tonight" regardless of whether I plan to share a pizza or eat a whole one.