The smallest searching the web is:
http://www.trango.com/carabiners_draws/SuperFly-carabiners
3.6 x 2.24 in. I don't know if it would work in your pocket. My smallest locker is 4 × 2.5 in and to do a test I put two sets of keys on it and put it into the front pocket of my 5.11 cargo pants. The thing was huge and not very comfortable. I would think you might rip jeans or slacks.

My suggestion if you wanted something strong and lockable would be:
http://m.rei.com/mt/www.rei.com/product/471288/petzl-oval-screw-link
It is much slimmer because there is no spring gate (3 x 1 in) but it is stronger then a regular carabiner.