Review on 🐱 Petsfit Expandable Cat Carrier & Dog Carrier - Airline Approved Soft-Sided Portable Pet Travel Carrier for Kittens, Puppies - Removable Plush Mat, Pockets & Safety Locking Zippers - Machine Washable by Michelle Meyer

I wasn't sure what would fit under the seat. We ended up trying the smaller 18x11 and it was great! It barely fits under a Hawaiian Airlines seat, so I was glad we used the smaller one. 19x12 would be too high. We ended up only using one of the two terraces because it was too big if we opened both. I highly recommend taking economy plus seats if you can, it gave us 6 inches more legroom and it made a huge difference on the way back. I've posted pics of getting there when I have NO legroom and with big seats in the back. My little girl is about 19-21 inches long, 12-13 inches tall and 13 pounds. She rode on our carry-on but I would recommend taking the Travelon Bag Bungee UTF8&qid=1465063434&sr=1-8&nodeID=9479199011&keywords=Backpack+Strap to attach the dog to the luggage handle. You can see it in one of my photos. There is a flap you can thread/secure the strap through, but I felt a lot better with the bungee. If you try to compress it, it won't do much. So if you're arguing about sizing, go for 18x11 as we really cared about an inch.