Frank, the trailer is 8 ft wide too, so you have to account for that dimension.
I just built an outside metal "carport" for my 1707T, and sized it 20L x 12W X 10H. It's actually about 12ft high in the center with the roof slope. I wanted to be able to raise the trailer inside the shelter. These dimensions seem to suit my trailer perfectly.
I needed the shelter because I've moved to Pine, in the mountains of Arizona and we get snow (and more rain) up here.
- Jack