Wrist curls will develop your forearms, and depending on your genetic structure, it may present an illusion of bigger wrists (depending how closely to the wrist your forearm muscle gets developed). Also it depends on age, and you will fill out over time. It is pretty difficult to develop your wrist itself. But take Bob's advice if that is your goal.
It is generally ideal to have small joints though - ankles and wrists, the reason being because if you do, it makes your legs and arms look bigger. It's like having a small waist - makes your waist to shoulder ration look bigger.
Small joints are ideal - Just bulk up the rest of you and you will look great - trust me. A bulked ectomorph is a thing of beauty - well proportioned but still elegant without just looking like a non flowing mass of muscle. Consider it a blessing in disguise