Sizing is not super-critical. Personally, I can use either a medium or a large about equally well. It’s more of an issue if you have large knuckles, just as ring sizing is with some people. Pick the larger of the sizes you’re between, and it works quite well to pad the inside of the Delrin ring with something. Tape isn’t bad; a cut section of a bandage is ideal. You’ll have to experiment a little to get just the right amount of padding. Brooks Story, the man who invented this slide, sets his up this way.