What size baking dish should be used?

FYI, I made this recipe with the quantities as written and the addition of a small bell pepper and spinach. I ended up with a couple of ounces less yogurt in the topping (because I forgot how much I needed at the store). The base fit just fine in the 9x9. When it came time to add the topping, there was just enough room. But the topping expands during baking, so I had a small overflow (like a couple of tablespoons). I though this might happen so I put a baking sheet underneath. So for others wondering, there you go.

The recipe suggests a 12" long pan, which makes me think of a 9x13" (lasagna) pan. That holds 14-16 cups or about TWO of 9x9" later cake pans.