The bearings must be fitted correctly to the axle, so the inside diameter of the bearing must match the outside diamter of the axle.  In the roller skating industry, there are usually only 7mm and 8mm sizes.  The 7mm skate axles require bearings with designations such as 627, while the 8mm skate axles require bearings with designation such as 608.

Quad roller skates usually have 7mm axles for indoor-only skates and 8mm for indoor-outdoor skates.  Inline skates and skateboards are almost always 8mm axles.