These nickel-plated steel O-rings can be used with many of our strap-on harnesses to accomodate a variety of dildo sizes. They also make an effective no frills, utilitarian cock ring.
The number in the pull-down menu indicates the inside diameter.
For example, Item A250 has an inside diameter of 1.25". (That's 1¼ inches.)
For a correct fit, measure the circumference of the erect penis (and balls if desired) at the place where you want the ring to go. Divide by 3.1 and you have a ballpark number for the diameter of a snug-fitting ring. (Don't go any smaller than that; you don't want to get stuck!)