The opening of your pet door should be 1/2 inch wider than the width of your pet’s shoulder. If you have more than one pet, the door should be wide enough for the largest pet, but should be mounted low enough so the smallest pet can get through it easily. however, it is not a good idea to buy a pet door larger than your pet actually needs. too large an opening leads to over zealous behavior and money spent on spare parts.
SD-300 Small - For cats or toy breeds up to 4 inches across the shoulders
MD-400 Medium - For Beagles and Poodles or pets up to 8 inches across the shoulders
LD-500 Large - For German Shepherds and Boxers, or pets up to 11 inches across shoulders
XLD-600 X-Large - For Great Danes and St. Bernards, or pets up to 14 inches across shoulders
Teaching your pet to use the pet door
With just a little help from you, your pet can quickly learn to use the pet door. Lift the flap slightly and coax the pet to come through a few times. give him lots of praise when he comes through. If the pet is resistant, a treat or his favorite food can be useful. If your pet is highly resistant, you can tie up or remove the u-frame portion of the rubber assembly until the pet feels comfortable using the pet door. then you can put the u-frame in its original position. Never push the pet through the opening as he may become frightened and continue to resist efforts to teach him to the use the door.