If your dog or cat meets the requirements listed in Southwest Airlines pet policy to travel in-cabin, you will need an approved airline pet carrier. The maximum size carrier that can be carried on a Southwest flight is 8.5" high x 18.5" long x 13.5" wide. If you are traveling with a soft-sided carrier, the height should be measured when compressed. There must be sufficient room for your pet to stand up and turn around in the carrier. It must have a waterproof bottom and must have adequate ventilation.