Depends on the bike and your riding style. Just like with a road bike, seat tube length is only part of the equation. Reach is an important consideration (and is dependant on the head tube and seat tube angles as well as the top tube length). Some companies have a rider sizing chart for each of their bikes on their website (Yeti and Transition for example). Eventually you'll get a feel for what bikes will probably fit you and what bikes won't by looking at the geometry, but for now, the best you can do is take a guess based on your height and inseam, and see how different bikes feel. How tall are you anyway?