You didn't give the caliber of your gun in your post and without knowing it, it is difficult to suggest a good load for it.
I can say that the Dixie Gun Works catalog gives some original loads for a couple of pistols somewhat similar to yours.
.67 Tower Flint Pistol = .650 ball/35 grains FFFg powder
.58 US Springfield Pistols Model 1855 = .570-.575 ball/40 grains FFg powder
That's about the best I can do except to say I would use about 1/2 of the bore size as a rough guide. For instance, a 50 caliber would use about 25 grains of powder. A 69 caliber would use about 35 grains of powder. The ball size would be slightly smaller than the bore and cotton cloth patches under/around the ball would be used to bring it up to a snug fit in the bore.