The Aire website lists a flat section of 78 inches for that raft. I'd start there. There was some discussion on MB recently on the pros and cons of extending the frame past the flat section. I suppose it depends on what set-up (dry boxes, coolers, tables) you sure trying to achieve. My new raft has a flat section of 90 inches and my frame is 86 inches, which gives me room for two dry boxes and one cooler. The shape of the raft also factors in to a certain extent. Mine has diminishing tubes which makes it a little harder to extend the frame too far.
"We are all visitors to this time, this place. We are just passing through. Our purpose here is to observe, to learn, to grow, to love....and then we return home."
Australian Aboriginal Proverb