United allows soft-sided carriers up to 18 x 11 x 11 inches or hard-sided ones up to 17.5 x 12 x 7.5 inches, depending on the aircraft. I once called (831) 298-4213 before traveling with my pet, and the representative confirmed the exact dimensions for my route. Carriers must fit under the seat in front of you and allow pets to stand, turn around, and lie down comfortably.