28 to 34 footers off road
I have never seen an Airstream while camped off the main road, other than ourselves.
The biggest problem with the Airstream is their clearances beginning with the axle(s) and then the plumbing. The majority of SOB (Some Other Brands) have much better clearances than the AS and more capable for the non level gravel roads.
The next problem with a longer trailer is the hinge point between the front and the rear bumper when encountering a "dip" in the road or a steep increase in road grade or steep decrease in road grade. If you cannot imagine what I am saying, when you encounter a dip that runs at an angle across the road, you will then understand why there are two "flimsy skids" beneath your bumper frame... well the 23 footer has one on each side.
Most of the main county gravel roads in the National Forests can handle all lengths of AS's. It is getting off the main road to a camp site coming off the road at an angle or with a slight culvert to cross. At times the access to a camp site has the pull out coming from the opposite direction, so you have to find a place to turn around and make the turn.
I find that some gasoline stations are more difficult to navigate than the roads in the National Forest. Asphalt is not as forgiving when you "drag your tail". Dirt and unconsolidated gravel roads can be lightly plowed.
You would imagine our 23 footer must be a wreck. It is not. When washed after a couple weeks of camping, you would not know the trailer had been in the remotest areas of the West. There are more obstacles on your street, gas stations or parking lots to avoid crushing aluminum. One season of practice will give most travelers the confidence to foresee problems ahead and work around them without incident.
I say "most" as some people haven't the slightest clue what they are doing and everyone else on the road is an idiot... And please, if you think I am picking on you, and I do not know you... you might give it a second thought before leaving the asphalt.