Hi
I had used the medium sherpa bag for my Jessie. My Jessie had to be put to sleep in February. She was almost 15 years old, so I know how you feel. Jessie was big-almost 15 pounds and the carrier was fine for her. However, I now have Jamie who I expect to be about 8-9 pounds fully grown, but she is quite tall and long. I plan to use the same carrier. It is the basically same dimensions as the Delta Sherpa. You can see all the products including the Delta bag on the sherpa website.
www.sherpapet.com
I've copied the dimensions of the bags from the website:
Small - for pets up to 8 lbs.
Model #55534
Dim: 15" D x 10" W x 8.5" H, Weight: 2.72 lbs.
Medium - for pets up to 16 lbs.
Model #55234
Dim: 18" D x 11" W x 10.5" H, Weight: 2.87 lbs.
Large - for pets up to 22 lbs.
Model #55514
Dim: 20" D x 11.75" W x 11.5" H, Weight: 3.01 lbs
They also show a wheeled version online.
I mostly flew Delta and they had no problem with the carrier's height since it could be swished down under the seat.
My Jessie loved to fly and regularly flew for up to 7 hours with no problem. Most people never knew there was a pet on the flight. Hopefully I will have the same luck with Jamie. She is only 5 months now so I'm giving her a little time to calm down before we attempt a flight. Best of luck!