One answer is “any dog that fits comfortably in a carrier that the airlines will accept for in-cabin pets.” As I’ve discussed in an earlier post, though, there are significant variations among the U.S. airlines’ requirements for in-cabin carriers, and there are lots of carriers that exceed those requirements in one or more dimensions but still sneak by for airline use. All these variations mean that the upper size limit for a dog that will fit under an airplane seat isn’t well-defined.
That said, I can tell you that our dog Chloe is pretty close to the largest dog that can fit comfortably under an airplane seat. She’s a Cavalier King Charles Spaniel, and she’s about 12″ tall at the shoulders, about 15″ long from shoulders to rump, and about 12 lbs. That’s pretty much breed standard for a Cavalier. Please note that there are many Cavaliers who are considerably larger than breed standard — they approach Cocker Spaniel size, in fact — and they would not be comfortable under an airline seat. Chloe can fit in a medium Sherpa bag, but is more comfortable in the large SturdiProducts bag she travels in (her bag is non-compliant, but so far it’s made it on board — and it’s marvelously flexible, so it works well even in limited and odd spaces).
U.S. airlines require that your dog be able to move around in her carrier — which I take to mean having enough room to shift position and curl up to sleep. The funniest dog travel sight I’ve seen so far was a mid-sized poodle mix making her way back into a completely non-compliant carrier after using San Diego’s airport pet relief area. Not only did the bag (a pastel Le Sportsac tote) fail to meet the usual requirements for a dog carrier, it was literally exactly as large as the dog herself. When she was zipped back in and waiting for her owner to get the rest of his gear together, the bag itself sat down and waited. She and her owner both seemed comfortable with the situation, but a little more room (and mesh ventilation panels! and a leakproof bottom!) is really necessary.