[EricH]

I believe you want to look at the "Basic Field Drawings" set. This will give you the width of each wall plate (72", per page 8--you'll need 3). Note that while there are supports between them, the plate meets up on the field side so the width doesn't count. Incidentally, the supports (not walls, just supports) are only about 2-3" square tubes, I want to say the latter.

72"x3=216", which is about 18 feet. Given the field width, you should now be able to work out the dimensions you need.