How tall a bike should you buy?

The old truth was that you should have 1-2" clearance between your crotch and the top tube, but current designs often use sloping top tubes whch messes up the old reference. There are plenty of sites that'll give you some guidelines, do a search for "bicycle fitting" and see what you'll come up with. Try a couple of the different methods and them pick a comparison from the middle of the group. The most crucial value is that when you're in the saddle you should be able to have your leg almost fully extended when the pedal is at its lowest position.