Our is 15X42. 15" wide seems to be the bare minimum to be able to open the door on one side and fully extend the slides on the other. I spec'd it too tall though.
It's an ugly addition to the landscape but it sure does a nice job of keeping the weather off our new TT (and we never use the back yard for anything anyway).


It was great doing projects on the trailer last winter. I can even wax it when it's raining outside!
(Sorry not a FW)