I know this has been discussed here before, but as far as I know, Airstream has always used the bumper to hitch length, generally rounded down to the closest even foot. For example, our '66 26' Overlander was actually 26'8" long and our current 25' Sovereign
is actually 25'11" long. Sometimes the "round down" may only be 4" or less as noted in the specs for current trailers on the Airstream web site.
This is a general rule of thumb. There may be a few cases where Airstream rounded up (although I am unaware of any at the moment) and there are a few cases where they rounded down 12" or more. Supposedly a 28' trailer from the 90's and newer are actually 29' long. And a similarly dated 30' is actually 31' long. And I believe there may have been some 34' trailers that were actually 35'1" or 35'2" long.
But in any case, I believe Airstreams were always measured bumper to tongue, while at least some other brands do specify the trailer length as just the length of the body excluding tongue and bumper. I think my buddy's 22' Prowler is about the same length as my 25' Sovereign