Student or Learner
Someone has once explained to me that when talking about something you eat (as in I like eating pizza), "pizza" is uncountable. But if you think of the whole pizza (as in I ordered 6 pizzas for the party), then "pizza" can be counted. But some pizza places have individual pizzas for 1 person. So you can actually finish one whole pizza. In this case, can we say:
I had a pizza for lunch.
(as opposed to "I had a hamburger for lunch.")
or do we need something like "whole" to emphasize the fact that an entire pizza was eaten?
I ate a whole pizza for lunch by myself.
Please be aware that I'm neither a native English speaker nor a teacher.
'A whole pizza' is meaningless; they come in so many different sizes.
If you told me that you'd had pizza for lunch that would be enough information for me. I really wouldn't care whether you'd had a small slice or a whole 16" job.