The "size" is the Inside Diameter. (The measurement straight across the center using a regular ruler on the mm side). The ring fits over the shaft and over the testicles/scrotum when a man is not erect.
When a ring is the perfect size, it is loose but stays in place when the penis is soft, but snug and can fairly easily slip off when the penis is hard. Naturally lubricants that are safe for the genital area can be used with this piece.

Our 316LVM Surgical Steel Cock Rings, strange as it sounds are not a self explanatory piece. The Cock Ring is usually worn to make the erection a little harder, larger, and to keep it a little longer. This will delay and heighten an orgasm. Cock rings work by constricting blood flow, keeping the blood in the shaft longer.

Since the cock rings are not adjustable you do need to order the correct size for a proper fit. It can be great, but you should not order it too tight, or leave it on for a long period of time, such as overnight.
They come in different inside diameters and also different heights (widths). No piercing is necessary.