Several questions.
Some short besses were smaller caliber. Most what we see are .75, but it depends on the maker
How short? I had a 30” bess
A hot charge 120-150 grains gives some boys real good results with bare ball. A 60-70 grain charge will turn Bambi French in range.
My start point would be a .735 patched ball, and 70 grains 2f. But…..
any big bore musket is a blank canvas. Load military cartridges with .690 ball, loose powder wad below and on top, loose powder wad just on top,715 or 735 shot, buck and ball, tow wads, proper wads, dog hair ( works good-stinks) endless playground.