Diameter (mm). Specifies the size in millimeters. = A typical skateboard wheel is between 52-58 mm. A typical longboard wheel is 65-76 mm. It is crucial not to take unnecessarily big wheels for your riding style for the sake of saving weight and avoid wheel bite.
How smaller the wheel how faster it will accelerate, how bigger the wheel the higher the top speed is.
If you want to know what fits best for you. Check more of the wheel guide, where we got a guide for every discipline of skateboarding there.