I am thinking that Mountain State Muzzleloading published a chart of ram rod size for most popular rifles.
The sub .40 caliber rifles mostly all used a 5/16 rod, CVA's and Traditions used an odd one, 23/64 (must be metric 3/8), and all the US made rifles over .45 cal were 3/8" diameter. Exception to that was The Great American Sporting Rifles were 7/16" diameter. However their 8 bore and 4 bore rifles just used old broom sticks.
The chart also gave you the length of the rods. When I was making ram rods, I found that info quite invaluable.
I don't recall ever getting any bulk 23/64" wooden ram rod material, but all the rest were available through various sources. Usually in 48" lengths. The cut offs made good pistol rods.
Hickory is the wood that is typically used for ramrods, although some import rifles use some sort of wonder wood that I don't think is even suitable for a fire. Miserable stuff. Replace the rod immediately. For the unbreakable ram rod crew, the most popular material is delryn, a form of nylon. I guess it is OK, but personally, I find it way too flexable. When viewed from a distance, it does appear to be ebony, so that's cool.
If you are putting brass ends on the rods, don't just glue them on, cross pin them also.
I found that for inletting the rod end, using a tubing cutter was an easy way to score the rod for a sharp, clean surface for the rod end to mate up to.
Although not terribly PC, a steel roll pin is a good way to pin the end onto the rod. Else use a piece of thin brass rod.
And there you are folks, Ram Rods 101. The accompanying text is Ram Rod Construction for Dummies, available at the company store.