10". You can by all new plates, hubs, pads, and wheel cylinders in a kit from Champion Trailer supply. Super easy job. 4 bolts hold the plate on, and one connection for the brake line. Just replace it all. Something like $110 for the pair.
New kit comes galvanized too. Don't forget to check your master cylinder fluid. If its rusted, just replace the master cylinder too. Otherwise, you'll corrode the new wheel cylinders.