Minie Balls:.577 or .58 cal.
Since you are new to muzzle loading rifles like me, you probably do not cast your own minies or round balls.
I have never shot a round ball in my rifle musket, and probably won't. I am relatively new to BP rifle muskets.I have been shooting my P53 Enfield 3-band, .577 cal. rifle musket with 1-72" twist rifling about ten times last September through November. The slow twist rifling in my Enfield shoots a short and light Moyers "58 cal." best. I use Moyers Lyman 575213-OS, .900", 460 gr. minie ball or DGW BU0901 Lyman 575213, new style, .950", 470 gr. minie ball. The difference in those two is the length, weight and depth of the grease grooves. The DGW new style minie grease grooves are not as deep and are less likely to blow off a skirt ring when it leaves the muzzle, destroying the trajectory.
A musket with a fast twist such as yours, would shoot a longer and heavier Track-of-The-Wolf Lyman 575213-PH, 1.06", 566 gr. minie ball or a Lyman 577611 (actually .575" OD)Confederate Gardner style (two grease rings), 1.00", 540 gr. minie ball using 50 gr. 3Fg GOEX or 60 gr. 2Fg GOEX.
All those minies size at .575" OD.
Faster twist rifling shoots heavier minie balls best and make those minies more stable in flight.