What size are shimano MTB (wheel) hub ball bearings? I wish to replace the Ball bearings in my Shimano hubs. What is the standard sizing for MTB?
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.