We flew with on Delta once and had no problems. They require a vet
check within a week of the flight, but no one ever asked for ours.
During a layover, we took her out of the carrier and one of the workers
said we could hold her, but that she couldn't be on the floor. Our dog
is short, but weighs 20 pounds. Our bag was the largest sherpa and no
one ever questioned the size.