So if you currently ride a 56cm bike with a 55cm top tube, but you know you can fit a 58cm frame, don't be scared off it just because it has a 57cm top tube. If you aren't sure whether your saddle is set at the right height, or if you just want another opinion, measure your pubic bone height. With your bare feet ten to twelve inches apart, measure from the floor up to your pubic bone. Hook a metal tape through a thin, hardcover book or a record album cover, and push up until you smash into the bone. Have a friend take the reading on the floor.
Sole thickness affects it, too.