Porcupine,
Take a look at the photos in this thread. http://tfguild.org/forums/ubbthreads.php?ubb=showflat&Number=16209&page=1&fpart=2
The bents on my frame are 4' on center, clear span is nearly 20' and the anchor beams are 8" x 14" white pine. I'm not saying these dimensions would work for you but it will give you an idea of the proportions.

Karl.