I have traveled a few times with Buster. I would be careful getting a bag too big unless it crushes down. Buster had a hard carrier that I had to place on it's side as it was too tall for Southwest airlines seats. They don't measure the bag but the bag does have to fit under the seat so the dimensions are important. Also the middle seat seems to have the largest dimensions. Honestly you don't want to be dealing with making arrangement to ship your dog when you are trying to fly out. Is there someone there that could care for your baby if suddenly you are at the airport and your bag does not fit. Also some airlines refuse to ship in the warm months as the hold is not temperature controlled so that is a concern too. I would see if you can check the bag out at the airport. I think you really need to make sure the bag fits so you all are not stressed at the airport. I also had to have a health certificate within 7 days in order for Buster to travel on Delta. Have a lovey trip and visit!
Pet Travel Requirements & Restrictions | Delta Air Lines
These are good carriers that compress but still look to be taller tan the 9" stated by Delta.
Sleepypod Air | Dog Airline Carrier and Cat Airline Carrier | Airline Approved Pet Carrier
Once you have the carrier keep putting your dog in it with treats so they are used to it before the trip. You may also want to look in to medication to help with anxiety whether prescription or homepathic as that is a very long trip.