My dog Chica has travelled to Mexico in her Sherpa bag (the black carry-on/backpack type) since she was 4 months old and she is now 4 years old and 12 pounds and still fits. It looks so much like a regular carry-on that I have never had anyone clue in as to what was in the bag. My seatmates have never guessed or realized until we land in Mexico and she gets as excited as I do! Sherpa's website has sizes and weights appropriate for your dog. The airlines websites also give measurements..but bear in mind if its an inch or so to big, they do flatten down well.