That really depends on your goals and your current starting point. For most people a box that is somewhere between 12 to 24 inches should work fine, but even that is quite a large range. If in doubt, start with a lower height. You can always opt for a box that has multiple heights depending on which side acts as the base (picks included in this list). The key thing really is to make sure that you are able to do quality reps as opposed to worrying about the height of the box.