There are quite a few different size frames out there, so here is a way to figure out what size bike fits you best.
1. Measure your inseam while barefoot. Stand against a wall and put a book between your legs so that it is pressed right up against your pelvic bone. Make a mark with a pencil on the wall along the top of the book and then measure the distance from the floor to the mark in centimeters.
2. Use your inseam measurement to get a rough idea of the frame size you will need. Do this by multiplying your inseam by .65. This will give you a good estimate of the bike size measured center to center. For example, if your inseam is 86cm, you will fit a 56cm frame (86 x .65 = 55.9). If you are looking at bikes measured center to top, multiply by .67.
3. Once you have the size figured out, test out some bikes that are that size to determine which is most comfortable to you.