One thing to remember here are that there are stretchy rubber cock rings and rigid rubber cock rings. A rigid rubber cock ring should be the same size as a steel/metal cock ring. A stretchy, constricting ring (such as a hair tie) or other stretchy rubber ring with constricting elasticity could be a quarter or a half inch smaller inside diameter to achieve the desired constricting effect; however, this type of ring should not be left on for long periods of time. A good rule of thumb is to wear a constricting ring for 30 minutes or less.
A rigid ring (rubber or metal) can be worn for much longer periods of time, as long as it is not sized too small. With a rigid ring, dick and balls engorge with blood to fit the ring and then the ring serves to keep some of the blood from flowing back out. To maintain a pump, kegeling is often necessary. With a stretchy ring, the ring pushes down on the base of the dick and balls and, depending on the tightness, keep blood from flowing in and out. The penis fills up with blood, it engorges beyond it’s natural capacity and the blood stays there.
squashdzl,
I am roughly the same size girth as you are. I wear a 1.75” inside diameter cock ring around both dick and balls in both rigid rubber and metal cock rings. If something between 1.5” and 1.75” were available in the U.S., I would wear that size. I find that the 1.5” ring is too tight to be comfortable and safe if worn around both dick and balls. I have, however, worn 1.5” cock rings around the base of my balls only.
I have worn a 1.5” super stretchy rubber ring around both dick and balls and find it to be effective and comfortable if not worn for too long.
I would recommend that, for wearing around both dick and balls, you try a 1.75” inside diameter rigid ring to start out with. When I first got into cock rings, I asked the sex novelties store clerk what size guys usually bought. The answer was that 1.75” is the most common size.
See my cock ring threads for more details.