I usually use .457" diameter in my Pietta Remington .44s...but .454 is usually the correct choice. You want to be sure that you're shaving a nice clean ring of lead all the way around the ball when you're seating it.
A larger ball also stays seated a little better than one that's too small. I.E. typically, a .451" will creep forward in all of the Pietta .44s that I've fired (Remington or Colt copies).
Unlike BlackEagle, I prefer real black powder over Pyrodex for some odd reason. The stink from Pyrodex just curls my nose hairs in the wrong fashion.
I have steel-frame Remingtons and I load em with 35gr of FFFg black powder. I usually use Ox Yoke Wonder Wads over the powder, under the ball. I have played around with using patch grease over the balls to seal em when I ran out of wads too...but it's a slimy mess.
35gr is a pretty stout load, so if you have a brass-frame pistol, do NOT use it. Stick with the "standard" 25gr load.