"Est-ce que tu veux manger la pizza qui reste ?"
Translation:Do you want to eat the leftover pizza?
So, is this leftovers, like stuff that you put in the fridge after a meal, or is la pizza qui rest that last piece of pizza in the box at the end of the meal? Leftovers v. remaining.
All scenarios are possible, including a whole, remaining pizza (you bought 3, one's left).
The least probable is (in my opinion) what's left of a (partly eaten) slice.