Save Some Money! Measure your Pet!
Do Not measure your pet directly
You just can't do it with any accuracy.
Open a door as wide as he needs to comfortably walk through. Use a treat for motivation. Measure that width. You need the flap to be at least that wide. It doesn't need to be wider.
Realize that the top of the flap should be mounted at, or better an inch, over his height at the top of his shoulder (the withers).
The lower the bottom of the flap, the better. In no case, should the "step-over" exceed 1/3 of the height at the withers. Pay special attention if your dog has a particularly deep chest or is low slung and adjust accordingly.
General, a good dog if ever there was one, weighs 60 lbs. He easily fits through a 7" wide opening. His pet door measures 8" wide.
He's 20" at the withers and the top of the flap he uses is at 22". The flap is 15" tall so he's stepping over 7" (that's the "step over") This pet door suits General just fine.
Since this panel pet door has a "user-adjustable step over" feature, we'll probably end up lowering the step over to 5" and the top of the flap to 20". That'll make it easier to step over when he's older.
If you look at General standing right next to his pet door, you'd say "impossible". But actually it suits him very well.
If we also had a much shorter dog, we might have required more height in the flap to get the bottom down lower. The taller flap, in turn, would probably be wider as well.
Look for "high aspect ratio" flaps to get more height for a given width.
If you have a tall, skinnier dog or a large dog sharing a pet door with a smaller dog or otherwise would like to have a taller pet door without having the pet door be wider than necessary:
A pet door with a "high aspect ratio" would be the Endura Flap.
So what is the point of all this? What are we really trying to do?
We are trying to make the pet door as 'accessible' as possible for your pet.
Here's why. The time will come when your pet won't be able to use his pet door. Maybe he's got arthritis. Maybe he's got hip problems or an injury. Maybe he's just old and can't move very well. But he's still alive and he's still your 'baby'.
All these years he's been going outside to do his 'business' and now he can't anymore. You really have a problem!
The point here is that you want that time when he can no longer use the pet door to be as far away as possible. And what that means is that you want to buy a pet door that will be easy to use not so much now but when he's old and not so agile.
Get a pet door that's not really big enough and he'll use it for many years. But when he can't do it anymore, that time will be sooner than it would have been if you had chosen a properly sized pet door.
So what is "properly sized"? Well, the top of the opening is at or above his withers so he doesn't have to crouch. The width is as wide as he is or a little wider. And the 'step-over' is as low as you can get it while still keeping the top of the opening up at or above his withers.
Incidentally, this is what 'high-aspect ratio' pet doors is all about. For a given width, you get more height which actually means you can get the step-over lower to the ground for easier access later in life.
Of course, there are situations that can change things. Maybe he's a youngster and you're not going to be in this house for very long. In that case, mount the pet door at the correct height--at or above his withers--and he'll be able to use a higher 'step-over' until you move. You can get a properly sized pet door at your next house.