I did 12 x 12 x 3 deep. Some like 14 x 14 boxes.
Mine are molded plastic parts bins and can be cleaned easily with a hose if need be. I do have partitions between each one and a 6" walkway along the entire front of them. They sit at 24" ht across from roosts that sit at 48" ht.
Mine were 12x14x4 and I think if my Light Brahma were any bigger she would end up just laying on the coop floor. 14x14x4 wouldn't be too bad. I believe my next boxes will be a long community style nesting area instead of separate boxes.