Since you can choose your drive wheel size, decide what belt speed you want and how fast the wheel will be turning, and pick a wheel size from that.
Drive wheel diameter in inches = 12 * (Belt speed in feet per minute) / (Drive wheel revolutions per minute)
You might be changing the wheel speed with either pulley or a variable speed drive or both, so use the maximum wheel speed and maximum desired belt speed for this calculation. I like a top belt speed of around 3600 fpm, which goes well with a 4" wheel turning at 3600 rpm.
Of course, it could also be that I like that because that's what I've always had, but when I had the chance to try grinders that ran faster I didn't really enjoy making my mistakes so much faster.
Steve