Being the nerd I am, I measured the diameter of my shaft and balls so I could get one that's tight, but not constricting - that worked out really well. I chose the 40 mm as I measured around 42 mm, and it's really good. I recently got a prince-albert, and I can't take that out yet. This keeps me hard even after-the-fact, so I can't easily slip out of it afterwards because of the piercing. The sizing is exact - like I've measured it with calipers and it shows as 40.0mm (±0.2mm)--full measurements below: ID / OD / Thickness: 40.0 mm / 51.5 mm / 5.75* mm *thickness outside the resolution of my calipers, so value is calculated.