I would probably go with 7/16" for a .54 if I were using a straight, untapered wooden rod. If you want a spare ramrod that might get carried under the barrel, the size would be dictated by the inside diameter of your ramrod pipes. In that case, you would want it small enough to slide in place easily, but not so loose that it rattles or slides out when you tip the muzzle down, and not so tight that it would swell up enough to get stuck in the ramrod pipes in wet weather.
If you use an untapered rod in conjunction with a muzzle protector, it would need to be of a size to fit the hole in the muzzle protector. I don't normally use one, although they are generally recommended.
I like tapered wooden rods, but that's one item that you just about have to make for yourself because it needs to be properly fitted to your rifle. To make a tapered rod for a .54, I would start with a 1/2" straight grained hickory or ash blank. There was a very good and quite extensive thread concerning tapered ramrods recently, on this forum. You could probably find it with a search.
Shoot straight and be safe! Let us know what works best for you.
Notchy Bob