The diameter of your whole package (penis and scrotum) will in most cases represent your cock ring size.
Here are some instructions for measuring your size:
1. Use a piece of string to measure the whole package; wrap the string underneath the testicles (where the scrotum meets the body) and over the base of the penis shaft (where the penis meets the lower abdomen).
2. The string should be pulled taut in order to feel a little pressure, but not so tight that it feels uncomfortable.
3. Mark and remove the string, then measure its length in inches using a ruler. Try not to round off the number; the more accurate your measurement the better the ring will fit. This number is the circumference of your package.
4. Cock rings are usually measured in diameter. To get this number, take the circumference of your package and divide that number by 3.14 (aka PI). The average diameter for a package is between 1.75 and 2.25 inches.

It all depends on preference.

If you enjoy the cock ring to be tight, get a smaller girth. Or vice versa.

I suggest you start out with a cheap cock ring, so you can figure out what you like.

I agree, try all the different sytles becuase you may not like how one fits