Bearings for skates are basically available only in one size 608 (diameter 22mm). Absolute minimum of skates have smaller bearings 688 (diameter 15mm). 16 pieces of bearings are need for one pair of skates. Accuracy of bearings is usually marked with norms ABEC, ILQ and certain number. The higher the number the more accurate the bearing. ABEC 9 or ILQ 7 are then more accurate than ABEC 5 or ILQ 8. Unfortunately, overall quiality is almost impossible to recognize, so the most appropriate indicator is then price.