You need to actually measure the true bore diameter of your barrel, and not rely on the caliber designation put on the barrel by the manufacturer. Use an inside caliper to measure the bore, and make a decision on what size ball will fit the best after you have that information.
With any smoothbore, you generally want a ball that is .015"to .020" Smaller in diameter than your bore size. Cotton patching can be expected to compress about 1/3-1/2" the measured thickness, so choose a fabric for your patch that, when wrapped around the ball, will provide a good Seal between the wall of the barrel, and your lead ball.
Remember that lead balls this size, cast from pure lead, will expand when fired, and help seal the bore, by pressing against the fabric hard enough that most pulled balls will show the weave pattern of the fabric on the sides of the lead ball. Fired balls that are recovered almost always will show the weave of the fabric patch impressed into the surface of the lead ball.
The primary function of that cloth patch is to center the ball in your barrel, and to carry enough lube in the cloth so that the bore is greased when its loaded, and again as the ball is fired out of the barrel. The grease in turn helps prevent the fabric from burning inside the barrel, and softens the BP residue that gathers on the side of the bore, so that the residue can be cleaned fairly easily.