I have been trying to get into long term chastity for almost a year off and on and could never seem to find a fit that didn't start burning after a few hours. Everything online kept suggesting that I needed a smaller ring and my pain was caused by said ring sliding around. I would order smaller and smaller rings but the results were always the same. This weekend I was playing with my cage and figured with it being more for play than real chastity the size of ring wasn't important and just grabbed the first thing out of my bag. It was the large ring which I had never used before because surely it was the worst culprit of this so called rubbing. Turns out that was never my problem. I have a fairly large sack and was apparently "bunching up". Since trying the larger ring out I have been locked up going on 3+ days now (only removing the cage briefly in the shower to clean everything), and I hardly notice the device other than when I start getting an erection. So if anyone else has had this problem, don't give up. Contrary to the standard wisdom that you find all over the internet you may actually need to go bigger
You're right that you may need to experiment to find what works best for you. Most men find smaller rings work best, but you do need to be sure that the fit doesn't cause problems. Glad to hear you worked out an arrangement that allows you longer lockups. Enjoy.