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.