Friction is a big part of the calculation. Do you expect to drive on grass or gravel? You may have noticed that 99% of all pictures of Segways show them on hard surfaces.
Speed is important for a balancing robot. It needs to have a software-controlled speed limit which is about half the maximum speed of the motors. This is so that it can 'catch up' to get the wheels back under you if you go too fast or downhill. That means you need to understand the RPM-per-volt characteristics of your motors too.