One person, two people, a full table
A single person ordering one meal is generally fine with a small or medium. Two people sharing usually want a medium plus a side, so there is something to pass around while the pizza cools. Groups of four or more are better served by one or two large pizzas than by several small ones, both for value and for table space. Large boxes stack neatly, slices are easier to share, and you avoid a counter crowded with small boxes.
If you are between sizes, think about leftovers and timing. A slightly larger pizza reheats well for lunch the next day, while two small pizzas ordered for four people tend to disappear before everyone has had enough. If you are unsure, ask the store what size feeds your group — the Rathmines counter answers this every evening.
Whole pie on the vitrine field, crust edge lit from the left.