How the math works
SnoutSmart mathMinimum flap width = chest width + 2 in. Minimum flap height = body depth (top of shoulders to bottom of chest) + 2 in. A door fits when its published flap is at least that big and your dog is within any weight limit the maker states.
LimitsFlap sizes are the makers' published openings. Install the door so the top of the flap sits at or above your dog's shoulder height, and keep the step-over low enough for your dog. Also check door or wall thickness and cut-out size on each product page.
Questions
How do I measure body depth?
With your dog standing, measure from the top of the shoulders straight down to the bottom of the chest.
How high should the door be mounted?
So the top of the flap is at or above your dog's shoulder height, with a step-over your dog can manage comfortably.
Should I size for a growing puppy?
Yes. Size for the adult dog; a flap that's too big is easier to live with than one that's too small.