You could write the vendor and tell them how much your dog weighs and take their recommendation. I see it is returnable if not correct.
I make coats and in my sizing, your dogs would be about 28-29 inches, depending. Some dogs are more compact than long.
I have attached a measuring chart for you to use. You could pick up a tape measure in the $store, probably. Don't make it too big. When your dog squats to poop, the coat will be hanging in his pile.
Good luck.
Measuring Your Dog.docx