An extra large dog house is one that is designed for the very large dog breeds, such as: Afghan, Bloodhound, Doberman Pinscher, Great Pyrenees, Greyhound, Irish Setter, Mastiff, and Saint Bernard.
Regardless of the actual breed, the house should be just big enough for your dog to enter, turn around, and lie down. Anything larger is at your discretion, but body heat retention during the winter could be an issue depending on the severity of the winters in your area. Bigger is not always better when it comes to dog houses.
Actual dimensions of a popular extra large dog house from Blythe Wood Works: 44"L x 44"W x 50"H. Floor size 41" x 41", Door opening 16"W x 25"H.
Source:
http://all-about-dog-houses.com/docs/term/extra-large-dog-house.htm
