Gosh, I dont even know what size mine are. I have two relatively new ones, that the liners were dropped, so I am back to using my smaller one. Kind of a pain. So I guess based on that statement, a large one
But I like to make extra to freeze, so I guess another factor is on how fast you can use it and how much you can store also.
Oh just re-read your post. 12 cups is 3 quarts, and then the bones also, and any veggies.... Even if you get a larger one, you dont have to fill it full. I always like to go bigger.