I've used 2-gallon square pots, and the yield was okay, about 3 ounces per plant. But the bigger your pots, the bigger your root mass, and the bigger your root mass the bigger your plant, and that generates more yield on a per-plant basis. You can do lots of little plants, or fewer big plants and get the same yield. If your city/state/country prosecutes based on the number of plants, I'd recommend being paranoid and having fewer larger plants...just in case.