Here's a barbell exercise that specifically works hypertrophy in your finger muscles, which are the only muscles in the wrist / lower forearm region of your arms:
I did them for sport specific purposes, and saw some improvement in my wrist thickness (as side effect). That being said, your wrists are like your calves from a body building standpoint - some people just won't be able to add much size. I went from finger curling 100lbs to 165lbs, and only had to wear my Timex maybe 1/4" wider, nothing crazy. But this is an exercise you can eventually go heavy at - finger curling your bodyweight for reps isnt an unreasonable goal.