I will be flying tonight with my dog on airtran. I asked them (at the airport ticket counter) if a 10" H soft sided sherpa bag. They said as long as it could squish under the seat it would be fine. Also the dog would have to stand up and turn around. I am not sure if they will unzip the bag and look to see if the dog can do that. My dog cannot stand upright in the bag but he can turn around and curl up at on end of the bag with plenty of room. I dont think any dog can fully stand up in these bags.
Also, we are flying from San Francisco to Atlanta on a red-eye. I think my dog will sleep the whole time anyway.
I will be posting my experience after we get to our destination.