Re: percussion handgun...
Assuming it is a 45, from your description , you could shoot either 2fg or 3fg. I would stick to about 25 grains. As for the size of roundball etc., there were 41's, tight 44 calibers probably actually 43, standard 44's, 45's and 50's. Spanish guns have been imported for almost 50 years and it could be almost any of those. Some of the difference can be taken up by patch thickness, but you are still going to need to measure the bore and subtract about 10 or 15 thousandths to get the roundball size needed. Do you have access to a vernier caliper? Even the fiberglass ones used by reloaders to measure case dimensions. Getting more accurate than a hundredth is probably critical. The rim diameter of a 357 Mag is .440 Try the base of the 357 in the muzzle. If it has just slight play it is a 44 or 45. If it doesn't fit it is something less than .44 (incidentally, an empty 357 case would make a good powder measure for the gun.
I f you have a 30-30 case, the diameter of the rim is .506 so it won't fall down the bore. Flip the 30-30 case around and slip the case in as far as it will go. The body of the case just above the rim is .422 ,, so if the case doesn't slip all the way to the rim, it is probably a 41 caliber.
So there, you have a way to measure the bore, without a fancy vernier.
If it is one of the old 41 caliber guns, you will need 40 roundballs and about .015 patches, if a 44 caliber, I think Hornady sells .427 roundballs and the same size patches. If it is a 45 caliber, 440 roundballs and the same patches.