Welcome to the forum! I would visit the nearest Airstream dealer with a good inventory and check out all the different sizes in person. From our perspective the following items are important filters for your eventual pick:
-- wet or dry bath (with stall shower)
-- kitchen layout and counter space
-- windows -- large opening with screen, or small Hehr sliding operable
-- mattress size/location/side access
-- single or double axle
The search function at the top, with the blue border, works very well and is powered by Google. You can enter quite a few search terms and get fairly precise results from inside this airforums.com site.
There are tons of threads comparing different sizes, so a search for "size 27 or 28" for instance yields:
https://www.google.com/search?q=size...=airforums.com
Good luck!
Peter