Mine is inside a rv garage/shop and the rv side of the building is 16' x 36' x 14 high (15' x 35' net inside measurements) this is the bare minimum width and still be comfortable with opposing slides, although we rarely put our slides out inside the building. I have a 12' wide door which is about right also. My building between the shop side and the rv side is as big as I can build in my development. I decided too that if I couldn't fit it inside then the rv is too big for us anyway (I had to remove our spare tire off the bumper to make it fit btw :B)
Dan