Everybody over-estimates the pet door size needed by their pet. If you don't spend 2 minutes measuring, you'll buy a much bigger pet door than you need.  Here's what to do:

1. Do Not measure your pet directly. You just can't do it with any accuracy.

2. 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 (though it's ok if it is).

3. 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).

4. 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.

Example: 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 up at 22". The flap is 15" tall so he's stepping over 7" (that's the "rise")  This pet door suits General just fine.

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.