"220,221,whatever it take's" :rotf
As in cleaning,there really is no "standard" caliber. If your question is whats commonly available commercialy,that to is subjective. As you've noticed, a few replies and it's obviouse that most everygun is different,my particular .62 Tulle' likes a .600 ball,and my .62 Mackinaw gun likes a .590,same patch and powder for both with 1" bench groups at 25yds.
The balls available in the south central U.P. are .490's. If that was a concern for me,I'd work up a load that's shoots that ball best,may take a lot of work,but you can get close.