The most common and standard bearing for almost all American roller skates are 608, with an 8mm axle hole. Some skates that come out of Canada or Italy or the UK are on the metric sizing and are 608 with a 7mm axle hole. Bearings are rated by the ABEC system. This is a system that rates the bearing on speed and weight under a load capacity. The ABEC system is not necessarily the end all be all for bearing quality. You will see all different prices for similar rated bearings. One major difference that it does not account for is bearing material. Bearings are made from different types of steel. There is carbon steel which is very soft and heats up at much lower temperatures, there are stainless steel, which is impervious to rusting, and chromoly which are the highest grade of metal bearings. Keep in mind, in roller skating, the higher grade metal is more important than the ABEC rating.