Now this is *subjective*. I'm new at Airstreaming but I'll add my .02.
My wife and I are selling our house and will be full-timing in our Airstream until we tire of it. Our goal is 1-2 years but we're flexible. I spent several months gathering information from friends who are avid RVers and from forums such as this one. After connecting the dots we decided on a mid-size trailer, the best we could find for our budget of $20,000.
We were very fortunate to find a '90 25' Excella in outstanding condition. Exactly the floor plan we like - twin beds in the back, mid bath and no dinette. We've only spent a few nights in it so far but we love it.
A bit of room to spare.
We find it's plenty big for us. We did a lot of tent camping when I was a park ranger, eventually "moving up" to a Westfalia. The AS seems large and comfortable to us. And I understand the mid size opens up camping opportunities that might not exist for larger units.
We wanted to use an SUV that we already had as the TV and set a limit on weight of 6,000. The older trailers are quite a bit lighter than the new ones - this one weighs 5,000 pounds dry.
I've been involved with the restoration, showing and racing of vintage cars for many years and I have come to appreciate old over new with many things. This is more expensive with the cars but fortunately, given our budget, it's less expensive with Airstreams. Vintage is sometimes defined as 25 years old which means our trailer is maybe just old at this point. But hey, it will be vintage in another couple of years.
Everything on our trailer seems to work well. It doesn't have some of the luxury items that are found on newer trailers, but I think it will deliver what we are after. Our main focus will be National and State Parks, boondocking, and only occasional stays in commercial RV parks. We're also interested in volunteering as campground hosts.
For us the key thing was to identify what we wanted to do with our trailer and to spec it out accordingly. We feel very fortunate that things converged on a satisfactory budget point.
Hope that helps.