We have a CAT flap on the shed so our cat can shelter from the rain and wind etc when we are not home, just recently my 2 cockers have taken a liking to the bowl of cat food inside the shed and will do anything to squeeze through the flap so don't worry tooooo much about size, I would go with whatever is recommended on the box, they usually state an example of the type of dogs it would suit. Our pets at home store have a lot of different ones of display so you can actually compare them for size etc.
Alsthough my Rosie is small for a cocker I still have no idea how she fits through the flap!
