You need to measure at the base of your penis, behind the balls. Use a string or something similar.
I prefer to start large and go down from there until you get the right fit. Buy the cheapest cock rings you can find. I think the 3 packs of rings they sell for harness work well for gauging the right size by trial and error. Then when you get a good idea of a size you are comfortable wearing, go ahead and purchase a nice expensive one.