What size are shimano MTB (wheel) hub ball bearings?

Question

I wish to replace the Ball bearings in my Shimano hubs. What is the standard sizing for MTB?

Answer 1

Rear hubs are generally quarter inch (9 per side, so 18) and front are generally 3/16 inch (10 per side, so 20). Obviously, if you're going to do this, order extras.

See this crib sheet from Sheldon Brown.