After reading your question, I would say, the problem is not with the size of the dog door, rather the problem lies with the doggie. The doggie, it seems, has gained a profound fascination with the wood of the dog door and thus it chews it. You need to train the doggie in the best possible manner in order to make him accustomed to the door and make sure that the doggie pal of yours, never ever chew the side woods of the threshold of the door, it is a bad habit and moreover, it is not cool for the health of your doggie since it can make the mites penetrate the system of the doggie and make the digestive system break.
Now, you should adopt the positive reinforcement technique, wherein, you have to make sure of the simple fact that whenever you find the doggie chewing the sides of the door then you shout at him – “No!”, but make sure that you never beat him up, because any type of non-punitive measures will not hold any good and will make the matters worse. Make the doggie understand that how he can smoothly cross the door, by taking a stuff toy and making the stuff toy cross the door the way you want the doggie to do and then doggie will imitate the stuff toy and may cross the pet door in an effective manner. And once the doggie crosses the pet door in a proper fashion then you should shower him with love affection, and treats (sometimes only). Good luck.
