There’s a measuring guide on Toys4NaughtyBoys where we get our metal cages from.
Honestly though, they’re such good value. Also they have a pretty generous returns policy from what I remember, and you can get different size rings for all of them so even if you get a cage you like but the ring needs to be bigger or smaller, it’s an easy change.
The way we did it was get the plastic CB6000 type first and use that to see what ring size suited hubby, but with the price and quality of these metal ones now you might want to just find one you like the look of.
The Arc and Tight Squeeze remain our favourites.