The medium PlexiDor is a perfect dog door size for a Welsh Corgi. The door is designed for dogs up to 40 pounds and the average Welsh Corgi weighs in somewhere between 25 and 30 pounds.
If you think the door seems small, cut out the opening size in a piece of cardboard and see if your dog seems comfortable going through. The opening is 9″ x 12″ and the door would be mounted between four and six inches off the floor. The top of the opening should be just above the dog’s back – they will duck their head down to push the door open, but shouldn’t have to crouch to go through.
Other breeds that commonly use the medium door size include Beagles, Pekingese, Cocker Spaniels, Dachshunds, and many Terriers.