I'm about 5'8" with a 30" inseam and ride a 21" or 54cm bike. But reading what the real geometry is... My favorite bike of what Trek called a 21" had a 52.2 seat tube with a 56.1 top tube.
When I ride a 22.5" or 57 it's big for my legs and the seat post is slammed.
Unlike some people, I'm not comfortable with not being able to touch the ground with my junk on the TT.