As far as I know up until 1968 Airstream
trailers were 7'6" wide, and then they changed to 8 feet wide. My 1968
Ambassador is 7'6" wide.
In the UK the law is that for a towing vehicle of less than 7500 kgs gross weight the trailer must not be more than 7'6" wide, and it can't be more than 7 meters long, not including the drawbar.
So the biggest Airstream you can tow in the UK with a 4x4 is a pre 1968
Overlander (26'). It will be required to have working brakes, so you will require a brake controller, or convert it to over run brakes which means changing the axles.