We upgraded from an '85 25' to a '94 28'. It sure felt like we gained far more than 3 feet of interior length. The 25' was nice, but a little constricting. The 28' is much better for us, but then a 30' with dinette looks good too. I can't see going to 34' unless we ever decided to full time.
As for the fuel economy difference, I would say that the three extra feet in length, extra 1000 lbs., and squarer shape of the '94 28' as compared to the '85 25' cost us only about 0.2mpg here in the Midwest. The fuel economy difference might be more pronounced in the mountains, but we have never towed there, so I can't confirm it.
BTW, when we toured the Airstream factory a couple of years ago, they said the current biggest sellers are the 19' and 25' models and in the the '80's it was the 34's and in the 70's the 31's. If I were to guess for the 50's and 60's it would have been the 22's and 26's.
Wally Byam Caravan Club International Historian
Vintage Airstream Club Historian