>>21462192 (OP)Pizzas are generally sold in Small, Medium, and Large sizes. Smalls are about a foot (12") in diameter, with larges being in the 16-20" range depending on who makes them. You can get a smaller "personal" pizza in the freezer section that is about 6" in diameter. This would apply to any flatbread style or french bread pizza of similar area.
Anything small and below is easily eaten in one sitting. The personal size is better if you want to maintain weight, while the small is for more active people.
When eating mediums or larges, you are supposed to only eat part of it. Maybe a slice or two, and that should be a meal. Depending on the style, 3-4 might be ok if they only ordered cheese or some shit. For parties it's best to take 1 or 2 slices until everyone is served, then come back for seconds to clean up food waste. If there is cake or other food, pick slice amount per desire.
And of course there are "party" pizzas, these are huge ass pies that get cut into many squares. You definitely SHOULD eat these with many people. There are some pizza places that do these, but not all.