The ball is0.312Is the clutch working? Why are you taking it apart? The ball could be stuck in the grease that's supposed to be in the hole. Sometimes it doesn't just fall out.
If the clutch is working, the ball is in there. If the clutch won't disengage, then maybe you lost the ball. It is a ball bearing, about 5/16" in diameter. Try a pencil magnet to see if you can remove the ball. If it isn't in the sprocket hole your clutch will not disengage.
Tom