We bought a size Large for our Corgi. Even though he has short legs, he has a large dog body. It fits perfectly because of his muscular torso. I only have one thing I would like to change about the jersey, which would be having the fleur-de-lis on each shoulder. (As seen on another Saints jersey on this website.) Because our Corgi is so short, the Fleur-de-lis and "New Orleans" on the front is not visible.
I don't know if the design on the front would be visible, even on a very tall, large dog.