I'm would think there is no set sizes as each bearing would have the balls picked to minamize the tolarences for each bearing. If i was going to do what your thinking of doing i would buy a brand new bearing remove the cage from the bearing and remove the balls. Then i would take them to the best quality bearing shop you know of that would sell high quality pricshion ball's for bearings and get them to mike them and let them hand slelect the ball to suite that pitcular bearing. There are 2 major probs with doing this your self you could make the tolarences too low causeing the bearing to tighten up when the engine is at running temp. Or the tolarences may be to low causeing excess movement. But befor you try id have an email to as many bearing manufacturs as posable like boca Tko acerbearing. and ask them some tech info questions. They should be able to ancer your question better. I have thought about doing it but your almost playing with fire an less its done properly you could distroy your engine. Let me know how you go Best Regards Jeremy.