Wrist size is primarily dictated by bone structure. People with a larger bone structure typically have bigger wrists. Thus, a person cannot affect the bone structure of their wrist, only muscle or tendon size. Muscles in the wrist are flexors (on top) and extensors (palm side) that extend into the wrist joint from the forearms. There are certain exercises a person can perform to develop both of these muscle groups and, in turn, increase the size or girth of their wrist. These exercises will help increase the thickness of the wrist when viewed from the side (width).
About The Exercises
There a number of ways to work the flexor and extensor muscles in the wrist to build size. You can use either a dumbbell or barbell. You can exercise both wrists together with a barbell or use a dumbbell and work them individually. Wrist curls and reverse curls are the exercises that work the wrist most directly from both sides. Wrist curls work the extensor muscles and are performed with the palms facing upward against a bench or your knee. These muscles are employed when you move your fingers and hand toward your forearm (flexing). Reverse wrist curls work the extensor muscles and are performed with your palms facing downward. Extensor muscles are used when extending your fingers or moving your hand away from the fleshy part of the wrist.
Start out with a dumbbell or barbell. It is best if you can use a workout bench that you can lean on for stability. But you can also brace your wrists against both knees; or one knee if you are using a dumbbell. Use a weight that enables you to do 10 repetitions. Do 3-4 total sets of this exercise.
Sit lengthwise a bench and lean forward with the back of your wrist (s) against the edge of the bench. Slowly lower the weight down with your hand (s) and stretch the forearm muscle (s), then raise the weight back up, flexing your forearm muscles. Repeat for the recommended number of repetitions. Be sure to stay within a range of motion that is comfortable for your wrists. Do not bend them too far down or up as you can injure your wrists. (See ref. 1 below for further details)
Reverse Wrist Curls
You will not be able to use as much weight for reverse wrist curls, as the extensor muscles are not as large or strong as the extensors.
Remain on the bench and lift the weight up and down with the palms of your hand (s) facing downward. You may want to place the dumbbell or barbell against your knees for reverse wrist curls. Using the edge of the bench can be an awkward position for some people. The other option is to kneel on the floor and do the exercise across the width of the bench. (See refs. 2 & 3 below for more information on reverse wrist curls)
You can also use hand grippers or squeeze a tennis ball to build bigger wrists. Do not overdo any particular exercise, as you can injure your wrists. Two to three days per week should be enough for any wrist exercising.