Absolutely on the comments above.
A hinged style can be more bothersome than it's worth. Clues will also be in the photos of the product as they will show hinged areas covered with rubber sleeves. You'll open yourself up to chaffing, nips & cuts etc. Also the misaligned issues when fitting and removing. That's just my experience very early on this path. But again they could be suitable for others.
If you have dabbled with cock rings, you'll get a feel for base ring style and sizes. But there are guides for measuring. And you can purchase base rings, or get knock off cages with a selection of rings to fine tune, before moving in a more expensive and 24/7 direction.
Having said that. One size doesn't work for all. Certain cages will have a dictation on base ring size. A good example is a Holy Trainer style cage, where it's lock is above the cage. One would need a larger ring size than they would think necessary for it.
Also one can vary in ring size on materials and what the purpose for being locked up.
I typically range from 46mm to 50mm , 55mm on a cobra baby style. 46mm daily use, I increase ring size for meets as I like to wear a strap and keep everything retained and restricted / held back.
Metal cages, 50mm / 55mm with a curved ring. Same reason as above.
If I'm going flat and inverted, needs to be 55mm and strapped back.
Holy trainer is a 55mm & 60mm.
So you may find over time, you may have a selection of rings for the cages you like to wear.
Placing a base ring on after a bath works well...but could risk accidently going too small, with the ease it becomes.
You just want the body to be warm down below and a very small amount of moisturiser.
If testicles are cold, that can be a bit difficult to get ring on.
Ideally you'll want to get your little finger inside the ring, under testicles. If you can't it's looking like it may be too tight...remember, at some point your going to be aroused in it , mainly from nocturnal erections. So every thing needs to be comfy.
But, you can always use a strap to pull it back.  .
Enjoy the locked journey.
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but if they physically won’t fit, please don’t force! 
However I’ve always used a bit of lube to help especially with the ‘snug’ fitting ones 
