Dog Door
I think I'd make it 15" wide x 20" high - like this one: https://www.amazon.com/Ideal-Pet-Products-10-5-15-Inch/dp/B0017JTV94/ref=sr_1_1?ie=UTF8&qid=1482166897&sr=8-1&keywords=doggie%2Bdoor%2Bextra%2Blarge&th=1
to me
And here's how to figure out the location of the dog door on the human door: https://lib.store.yahoo.net/lib/hightechpet/pet-door-mount-height.jpg
