What they are basically saying is they are going to wind a 21' pipe and then cut it in half. We cut them in to 10' 6" pieces first and then wound them. It works either way, as Jack says above you'll just have to heat one end and straighten it. The smaller the pipe is the easier it will be to assemble. We used 1/4" pipe and it was like a big spring that could be manipulated with a pipe wrench. 1/2" pipe would be not so easy and the closer it can be rolled in to usable form, the better.
I guess that's what we'll do, over wind 1 turn and heat and straighten the "tail" the drawing from reliable calls for 3/4" sch 40. I guess I'll have to eat my wheaties that morning.