I'll repeat some advice I've offered on similar threads in the past. First off, an adjustable cock ring is a good place to start as you can experiment with how tight a fit you want etc. For a solid ring I always recommend that a guy get an assortment of inexpensive rubber O-rings in the thickest available cross sectional diameter (typically 3/16") from a tractor supply store or industrial supplier. They're dirt cheap, about 20¢ apiece, and work as well as the similar $5 product you'll find at a sex shop. That way you can experiment with getting in and out of a solid ring and know exactly what size you'll want if you go with something fancier and more expensive.
A lot of guys (myself included) find the rubber products to get a little uncomfortable when worn for an extended period of time. That, because the cohesive nature of rubber grabs at the skin and can cause chaffing. And some people can have skin reactions to long term contact with rubber. But a relatively cheap alternative to rubber is to buy Teflon O-rings which, of course, are very smooth and slippery so don't cause any chaffing. The Teflon O-rings cost a couple dollars apiece, but once you know exactly what size you're looking for (from experimenting with the cheap rubber version) you only have to order a few of them. They can be a little harder to find but you can track them down on the 'net, I got mine from Rocket Seal in Denver (
Rocket Seals). I like to gang two or three of them together so's to get the wider effect like some of the high dollar cock rings.
With both types of O-rings, if you ever get into one that is so tight that you can't get it back off, you can safely snip them off with a pair of nippers. The smallest solid cock ring I can get into is one with a 2" i.d. So when I decided to get a heavy stainless steel cock ring I ordered one in a 2-1/8" i.d. just to make damn sure I'd never be stuck in it. Anything you can get into you should be able to get out of, but I figured I didn't want to take any chances with something that solid. And it's tight enough to serve the function.
Cock rings work principally by partially restricting the return flow of blood out of the penis. That makes for a stronger, harder, longer lasting erection. Also by maintaining pressure on the perineum directly behind the scrotum they do add some sexual stimulation to the wearer of the ring, though that no doubt varies from guy to guy. Since the cock ring itself never gets anywhere near the lady's genitals it doesn't directly stimulate her. But a tight fitting ring will definitely increase the penis' girth which can certainly increase the vaginal stimulation. I don't know about glans rings and/or shaft rings (never have used them) but they might well add to vaginal/clitoral stimulation as well.