1: hamlet (>100)
2: village (100-500)
3-5: small town (500-1500) (Trunau, Otari & Sandpoint)
6-7: big town (1500-5000)
8-10: small city (5000-10000) (Magnimar)
11-12: big city (10000-30000)(Port Peril according to the book, but I would rate it higher)
13-16: capital city (30000-50000)
16-19: major capital (50000-150000) (Katapesh, Shadow Absalom & Westcrown)
20: (<150000)biggest city in my world, comparable to twice the size of Absalom, but since Absalom is the largest in Golarion, I would rank it 20 as well.