In that case, make the pen as big as possible or portable, or you will have them in a mud pit pretty quickly.
I would not give them dog food. Geese do not need the animal by-products that dog food has, or all that corn either. I still don't understand why people think dogs should eat that much...
If its only them, and you have a predator-proof fenced yard, I would not pen them at all. Just have a safe, dry, warm house where they can nest and sleep.