As was suggested, measure your pistols bore, land to land. The ball size should be .010 to .015 smaller than the bore size.
Although patching with thick tight fitting patch/ball combinations is a good idea with a rifle, it can make loading a pistol a bit of a chore.
For that reason, you might want to start with some .010 thick patching (cotton only).
If the recovered .010 patches show that they have torn or burned thru try a little thicker patch.