Generally speaking the Pedersoli and Uberti Colt 1851 Navy and Colt 1860 Army clones use a .451" to .452" ball. The 1858 Remington clones are more likely to use a .454" ball and the Walker Colt and Colt Dragoon copies more often than not use a .457" ball. The Ruger Old Army was designed for a .457" ball.
From a safety perspective, if you try to insert an oversize ball, such as a .454" ball in a .451" pistol, you'll just note the greater pressure needed to seat the ball as the sharp edge of the cylinder trims it down to size. If you try a .457" ball in a .451 pistol you'll note it's extremely hard to do, and if you persist you'll run the risk of breaking the loading lever.
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Ideally, you want to shave a small even ring from the ball when seating it. It ensures a snug fit that will prevent the balls in the other chambers from moving forward under recoil, and in conjunction with some crisco, spit ball, or other lube over the front of the cylinder helps prevent flash over between cylinders that can cause a chain fire.