How do I know what size dog door to install into my home? Good question. We manufacture to sizes of dog doors. Small dog doors have a pet flap opening that is 6 x 10 (inches). Large dog doors have a pet flap opening of 11 x 17. There are many methods other manufacturers suggest you use to determine the appropriate size pet door to install for your dog. Some suggest using the dog breed to determine the right size dog door and others simply base it on your dogs weight. Those can be very vague barometers to use so be careful when using either of those methods.