Yeah, I suppose you'll just have to experiment to figure out what works best for you. Steel is the only way to go for long term use. But taking those two thoughts together for a moment: there are many plastic cages out there that come with multiple base rings. Maybe while you're experimenting anyway, one of those would be good, just for fitting? Then when you have it narrowed down to the best size, get a metal version?
And yes, it's possible to learn to cum in a cage. If you condition yourself to do it, it typically doesn't bring a lot of satisfaction. It's a bit like a ruined orgasm, if you've ever experienced one of those. You realize, "oh my god, I'm about to cum!" And then you unceremoniously dribble some goo and feel generally sheepish about it, and are left about as frustrated as you were before (or more). The only exceptions I can think of have been:
- My wife has tickled and played with me between the bars of my cage until I came. I obviously couldn't get a full erection, but as excited and engorged as I was, I can't say I was "flaccid" either. I was a hard purple mass of ultra-sensitive flesh squeezing out of the cage on all sides. That one was still humbling, because of how we were playing, but it definitely felt like a full orgasm.
- Once, during a particularly long lock up period, I was so connected with my wife's arousal and erotic energy while going down on her that I climaxed right along with her when she came, still locked up, with no stimulation to my dick at all. It was crazy intense and emotional and lasted what seemed like forever and left me feeling ******* and euphoric and overcome with closeness to her. It was absolutely the most intense orgasm I've ever experienced. My legs were shaking as much as hers were, and we laid there together, sweating and panting and holding each other afterward while time stood still. I've been close to having it happen again, or maybe I've had other sympathetic orgasms, but not with quite the same level of intensity. I haven't quite been able to go soaring blindly over the edge with her simply from giving pleasure her like I did that night. I remain hopeful and plan to keep trying though! I'd love for it to happen while she's being fucked by a bull, too, squeezing my hand or digging nails into my shoulders and looking into my eyes as he sends her into a screaming, gushing frenzy.
Sorry, I know I'm overly verbose. This is a topic I'm given plenty of time to think about, having spent weeks and even months without orgasms over the past 11(ish) years.
I say start buying cheap cages and adjust as you go. Communicate with your wife and have fun! Chastity play has been a rewarding enough adventure for us that it's the only thing about our sexual journey together that's lasted this long. We've taken a few breaks here and there, but my wife doesn't like to let me out often. And despite the frustration, I don't want her to give me back control of my orgasms. Not when I really think about it and what that would mean. A good, loyal dog might enjoy being off-leash once in a while, but have you ever met one who wants not to belong to his master anymore?