@edprof I took my 12 lb Chihuahua from California to Toronto on Air Canada. Lying down, she is fine. She can't stand up under the seat. She was a real trooper for the 6 hour flight (and 2 or 2 hours in the airport, where we (against the rules) let her walk around because every other dog was also walking around!)
Standing up, my dog is 15 inches tall to the shoulder, the bag is 12 inches tall, and the seat space is less, but the bag is soft and squishable. Our bag also zips out one side to make the interior space wider.
Before taking her to the airport, we went to the dog park and ran her around to tire her out for the trip. We had prescription trazadone (and some cannibis oil) but didn't use either.
Near the end of the trip she was super anxious and we sat on the tarmac for over an hour!! Luckily in Toronto, the custom's area has a dog relief room, we barely got her there and took her out of the bag when she peed. It's tough for a dog to hold it that long!
She was good for most of the trip. I recommend one of those dog bowl/water bottles. But you don't want too much water (because of the pee thing).
Whining under the seat was dealt with with some treats and putting your hand into the bag to calm her. She needed a blanket too as it can get cold under there.
The best thing is to have a really tired dog. A red eye might be good unless the dog gets a little anxious and other passengers are trying to sleep, however you are in a plane and the engines are loud so what's a bit of yipping?
