Wheels are measured by Diameter (Width) and Durometer (Hardness). 50mm to 54mm with a hardness of 99A – 101A is the most common size wheel for street and park skaters, while 55mm to 64mm is typical for bowl and old school skaters. The larger the wheel, the longer the roll. The softer the wheel, the smoother the roll. Softer wheels are great for rough terrain and are also quieter (which is why filmers prefer large, soft wheels). Ultimately, it comes down to personal preference and the type of terrain the skateboard will be ridden on.