Wheels For a nice diagram of proper wheel setup, see my bearing spacer page . Usually you will be concerned with wheel size and wheel hardness. Wheel size is normally referenced by noting the wheel's diameter, in millimeters. In the old days, it was common to have wheels with diameters anywhere from 60-70mm. Freestyle wheels were usually about 57mm.
Larger wheels seem to retain speed better than small ones, but they also accelerate a little slower.
Larger wheels also roll over cracks, rocks, and other junk a little easier. Of course, larger wheels are also heavier, a little more sluggish, and make the board ride a little higher off the ground. Small wheels are lighter, quicker feeling, get stuck in cracks easier, acclerate faster, and retain speed worse. They seem to be a good compromise between the two extremes. The higher the number, the harder the wheel.