for a standard cross country bike, you should have about 1-2" of clearance between your crotch and the top tube when you're standing over the bike. when sitting on the seat, and with a crank arm in the full downward position, your heel should just graze the pedal with your leg fully extended and your knee locked. If you cant get your seat post high enough to give you that, then you may need a longer one. The frame size and the seat post length are usually matched up pretty good. Other than these guidelines which are for optimal efficiency and comfort while riding, its all preference.
