Sometimes when I visit a shop they simply list flat white. Other times there are many many options for size. What is the correct way to serve a flat white?
It is common for a shop to offer only one size of flat white because a flat is a very basic drink. It's a shot of espresso with foam added. That's all.
If you further dilute a flat white by adding milk, you have a latte. If you multiply a flat white by using two or more shots of espresso, you end up with two or more flat whites.
If you run a shop and the flat white is a popular drink, add a double size and see how it fares. Go from there based on product performance. I'd not go more than a triple though because three shots of espresso in one drink is getting a little bananas.