5/16 steel rod would be more than sufficient for a .54. Or for most anything, really.
For a wood rod, I would think 3/8 would be adequate, as that is the size of most undergun rods. For a range rod, a 7/16 is a good choice.
I loaned my wood 7/16 rod to someone to use decades ago. Of course, he was able to break it. Still have it but don't use it now. I glued it back together and for me it works fine, but I would never loan it out again. Years ago, I worked in a chain saw shop. From seeing the things people brought in to have repaired, I developed a motto of sorts. Never loan out your truck, your chain saw, your tools, your guns or your wife. You will not get any of them back in the same shape you loaned them.
Gotta go,
John (Bigsmoke)