This is a cock ring sizing system designed to fit without too much pull but with enough restriction to be truly effective.  The Xact-Fit rings are only 0.1 in. (2.5mm) apart in sizing to get rings that would fit perfectly.  They are not too stretchy, so the rings are effective, but easier to get on and off than a metal ring.

The Play Zone kit contains the nine most used Xact-Fit sizes for shaft rings, ball rings, and cock and ball rings. Made of a plush soft touch silicone, each Xact-Fit ring is premium quality and incredibly strong.  Each ring is stamped with its size. Additional rings can be purchased. The Play Zone comes with a storage cone designed specifically to hold the rings perfectly.  The complete set of sizes available are shown below.  The Play Zone comes with PF Size #11-#19 (1.1 thru 1.9 in. diameter).