Skateboard wheel sizes normally range from around 50mm - 60mm, anything larger would usually suit cruiser or longboards.
The most common sizes are 52 - 56mm. Below gives an idea of what kind of terrain each wheel size is best suited for:
48mm - 53mm: Ideal for street skating
53mm - 57mm: Great for bowls, park and street skating (all-rounder, most popular)
56mm - 60mm: Ideal for vert and bowl skating
59mm - Bigger: Best for cruiser and longboards
* These guides are subjective, they are based on general opinions from experienced skaters