I see this is an old thread, but I'll chime in. I have a Large Sherpa on Wheels, 19x11x11, and I've flown my 24 lb corgi many times on mainline UA flights..... 752, 753, 320, without any problems. On smaller aircraft we use a Large Sturdibag. It flexes down a lot better than the Sherpa, which is showing signs of wear now...the zipper is hard to close because of all the bending of the frame due to "squishing". My dog can still easily stand and turn around in the Large Sturdi (18x12x12), he is a competition agility dog and very flexible and athletic. A few times a TA or GA has raised an eyebrow, but has always backed down when they see him turn in the bag.

What I really wish existed was an intermediate Sturdi size...The XL is way too big, but I'd like something an inch or so longer and wider than the Large. Or, that the Sherpa's frame around the square ends was as flexible at the Sturdi. That would be beautiful!