Years ago, when I was kickboxing, I would take 2, 5lb ankle weights, velcro them together around my forehead, lay on my back, and do neck curls (chin to chest) to failure, 3 nights per week before bed or after sparring. It definately made my neck bigger and stronger, and able to take a good straight punch better.
If you do deadlifts and squats though, and eat to bulk, you will gain neck size just from the stress placed on the neck extensors and traps.
Zig