My Rv half of the barn (other half is my 24x26 heated shop) is 15'6" x 35' inside with a 12'9" high door that is 12' wide. If I could I would have built it bigger but I am stuck with what the HOA allows for square footage and height. Our current unit clears the door for height by 3" on the 2nd AC unit and I have about 8" to spare on the length and that's after I removed the spare tire off the bumper.
I figure if it won't fit in the barn it's to big anyway lol. If I do decide to bigger someday I will store it offsite somewhere but in reality the next one may be shorter for ease of picking campsites and a little more nimble.
Dan