Just looking at the REI site again, you might try this one: http://www.rei.com/product/847491/edelrid-hms-strike-slider-automatic-locking-carabiner

I have no experience with this particular model, but it is brand new state-of-the-art,rather expensive, and advertised on the internet, so it must be good....

I am not at all sure that you really need the locking feature for a key ring. Dimensions are rarely, if ever, given in most descriptions because biners are a standard size, just large enough to accommodate a climbing rope and a sling or two. You will have to accept a biner that is about 4" in length. I wouldn't even consider the smaller toy biners. In my opinion, they are quite dangerous. Someone is bound to use them in a critical situation, with disastrous results.