Depending on the wheels, usually, all the wheel bearings come in the same size. A general rule of thumb, all wheel bearings have an 8mm inner and 22mm outer diameter with a 7mm width. In the inner race parts, there is a slight difference ranging between 7 and 8mm. All in all, the size of the exact bearing mostly depends on the axles that you put with the bearing or wheel. So first focus on the size of the axles and then take further decisions. A single wheel requires double bearings.