New to This
Sizing can be a challenge, especially if you cannot shop in person!
After initial experiments with hairbands, elastic bands, clothing elastic and toilet paper cores, I found a rubber o-ring set (mine came from Aldi's centre aisle) allowed me to experiment with all possible sizes. Since then I moved to stainless rings I found at the hardware store (near wire ropes and chains) of 40 mm (for the package) and 28 mm (glans).
If I was ordering a speciality item, they would likely be 40 mm (or 1.5") for the package and perhaps 25 mm (1") for the glans but I know others have said they must other a different size to properly fit.
I would recommend kingrings.com, they only do metal rings but they sell a 3d printed sizing kit for cock rings and glans rings. Once you buy this kit, they apply that price as well as a decent discount for ur first purchase. Or you don't buy anything but the kit, no strings attached. I love there rings, I have 7 of there cock rings and 4 glans rings, and I want more lol.