I've read that there are many exercises you can do to strengthen your "wrists". Grip is just part of it. Wrist curls (BB and DB), reverse wrist curls (BB and DB), hand grippers (captains of crush), pinching plates, wrist rollers, etc. I think pretty much every exercise you do that involves you holding onto a bar of some sort will increase your forearm strength which in turn affects you "wrists". I keep putting wrists in quotes because, as others have said, you don't actually build up muscle around your wrist.
I don't know for sure but I think the only way you can get bigger wrists is by fattening up and that's probably not your goal.