Dog doors must be the correct size for your pet; otherwise they can be at a high risk for injuries. Measuring the dog from his shoulders to the floor and the width of his widest point will give you the correct measurements for the appropriate door. Pets should not have to stoop low or step up when using the dog door. Common injuries that dogs can receive from a poorly measured door include scrapes, bruises, fractures and even broken bones.
If deciding what size dog door is best for a new puppy, use measurements for the dog door that coincide with what size the puppy will be when a full-size adult. This will decrease any chances of having to continue changing the door as the pet grows.
Measuring the Father (if possible) is the best way to determine what will be your puppies largest grow size to fit the correct pet door size.