In the US, pizzas have basically been standardized by the national chains to be 10 inch (25cm) for a small, 12 inch (30 cm) for a medium, and 14 inch (36 cm) for a large.
Honestly, this is probably the most effective way to communicate circle sizes to me, as I am quite familiar with exactly how big each of those sizes are.
I'm obviously familiar with other lengths as lengths, but for flat circles, there's not really a better intuitive comparison.