Interested in Language
'Pizza' is a countable/uncoutable noun.
When we are saying 'I am eating pizza', is pizza an uncountable noun here?
Does it sound natural to say 'I am eating a pizza' or 'I am eating a slice of pizza'?
You can just say "I'm having pizza."
Some pizzas are "personal sized" and you can eat a pizza. If it's a large pizza where we usually have a slice or two, then you can eat a slice of pizza.
They're all okay.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.