RE: What size round ball for .44 Navy pistol
Buy some .454 and some .451 roundballs and try them. A thin ring of lead should be sheared when they're pressed into the chambers. Shoot both and see which your gun prefers. Hopefully it will prefer the .451 because shearing that thinner ring of lead is easier loading than shearing a thick ring of lead.
When you'vefound which you and your gun like you can buy the respective mould.