There are too many different sizes to be able to answer this. Different manufacturers use different sizes and it also depends on where the bearing is on the bike.
The important thing is to get the right size. If you are not sure of the part number, a bike shop should be able to help you. Otherwise, the important dimensions are the inner diameter (bore), the outer diameter (O.D.) and the width.
After that, does it have metal shields or rubber seals? Do you have a problem with rust and want a stainless steel bearing? Does the bearing have a flange on the outer ring? The more details, the better. Choosing bearings for any part of a bike is important, but the most performance gain to be had, undoubtedly comes from choosing quality bearings. If performance is not important, however, cheap bearings will get you from A to B just fine!