IF it a gen-you-wine original, it will use a .457 round ball. Reproductions use the smaller .451.
Use 30 grains of FFFg or Pyrodex "P" behind the ball or conical. Do NOT use a patch or Wonder Wad! Patches have a bad habit of leaving themselves in the forcing cone, (as with modern revolvers, the barrel bore is smaller than the ball or bullet) and Wonder Patches are known for seperating from the ball, leaving a gap between the ball and powder. As one of the other folks said, the proper size ball when loaded will be shaved a little bit when rammed into the cylinder. This does seal the front of the cylinder, preventing cross-fire. It also will not hurt to put a bit of patch lube or revolver grease over the chamber mouths BEFORE placing the caps on. UNCOOKED Cream of Wheat cereal also works quite well for this. The nipples are interchangeable with modern made, and will use either a number 10 (most common size needed for them, and thus the hardest size to find) or number 11
Dixie's catalog is highly recommended, it has all information you need to determine if what you have is an original or an artificially aged reproduction, Serial number location and range, barrel script, proof marks and their location, Military Inspector marks and charteuses (if any) so on. (The also have a list of who the inspectors were, and what marks they used. Each inspector had an exclusive mark.)
Faked "antique" Remington 1858's are a problem, although not as frequent as some of the Colt open tops and the rare Confederate arms like the LeMat, (if you find an "original" LeMat for sale, it is either a reproduction OR one of the FIVE known (as of 1992) originals, stolen from a museum (two of the five known are in the West Point Armory Museum) and some of the Confederate brass framed jobs.