Wheels are available in different diameters. The width of a wheel for inlineskate is always 24mm.
The larger the wheel, the better the speed. Speedskaters mostly prefer wheels from 90mm up to 125mm. The typical wheel size for a recreational skater is 76mm to 90mm. Smaller wheels are recommended for beginners (76-80mm), hockey players (64-80mm), stunt-, free and urbanskater (50-84mm).
The hardness of a wheel will be measured in A, from a soft wheel in 72a up to a very hard 101a.