Wheel size contributes to speed, grip, and agility. In general, larger wheels are used for downhill and smaller wheels are used for cruising.
Larger Wheels: 71mm - 106mm
The larger the diameter of the wheel the faster it will be. These wheels can roll over sidewalk cracks or debris easier.
The Happy Medium: 69mm - 70mm
70mm wheels are the most popular. They are large enough to to provide a smooth ride and small enough for agility. If you have rough pavement where you skate, a 70mm wheel will help reduce the vibrations and chatter.
Cadillac Cruisers 70mm 80a Pink Wheels
Smaller Wheels: 60mm - 65mm
Campus cruisers and sidewalk surfers use smaller wheels for maneuverability. They also have less resistance to sliding, making them an option for freeriding.