Single speed machine, or variable?
In general, heavier grit belts (36/40/50/60) cut best at higher speeds. As the belt grit becomes finer, slowing the belt down is more efficient and more importantly, generates less heat then running them at a faster speed.
SOME abrasive belt producers will list "optimal" type information on their sites for specific abrasives (usually the structured abrasives), but there are so many types of material being ground, under so many varying circumstances, most will only mention general parameters.
In a single speed grinder application, I would personally prefer a smaller drive wheel to keep speeds down when I get into the finer grit belts. I'd rather have to spend additional time on a heavy grit belt, than burn something up when I get to the finer grit belts (because the belt is moving too fast). Of course that means you're not going to remove material as rapidly with the heavy grits, but just about everything we do in knifemaking is some type of "trade off".
If the machine is variable speed, the main consideration is the HP size motor being used, and the torque it generates......while a larger drive wheel will increase belt speed, torque drops off correspondingly as the drive wheel size increases.
If you're in the early learning stages of grinding, I highly encourage using slower belt speeds.... it not only helps in the learning process, but when a "mistake" is made, it's generally repairable, versus being beyond repair if the same "mistake" were to occur with the belt moving faster.