I'd say anything from 15 to 40 grains would be safe.
Personally, I wouldn't go as high as 40. Maybe 30 max?
The thing is, black powder takes a while to burn and in a short pistol barrel you won't get a lot of change in velocity by loading large powder loads.
Don't the larger powder loads kick more? Sure they do, but a lot of that increased kick is because of the additional weight of the powder charge.
Oh, that 30 grain max load will develop a muzzle velocity of around 1100 fps with a patched .440 diameter (128 grain) ball.
That's comparable to a lot of modern guns.