You really can't increase your wrist _size_. There is really not much there but bone, ligaments and tendons. But you can work on you _grip_. I have teeny tiny wrists too and my grip is a bottleneck for me, so I use gloves to help me out with that and work on my grip when I'm less concerned about working large muscle groups. And even occasionaly do wrist curls and raises. I have never been able to bring myself to get one of those springy gripper things. Too many mental images of overblown machismo.
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Keep your body lean, your blood clean and your mind sharp. -Rollins