What diameter round ball would you use in your ASM Black Powder cap and ball revolver?

It would depend on caliber, for a .44 caliber you would use a .441 to .445 ball, for 36 cal use a .362 - .365 ball or conical bullet, You want to have your ball slightly larger than the chamber size so when seated you shave a thin ring as the ball is rammed into the cylinder chamber thus sealing the powder charge from moisture and cross fire and giving the best performance with your load.