The biggest aperture "backpack-able" telescope that I know of would be something like the SkyWatcher 130 mm (5") collapsible table-top Dob, which collapses down to just 37 cm long:
http://shop.australiangeographic.com...ope-P2006.aspx
Other options might include a short-tube refractor or a Mak with a light-weight tripod (Meade make an 80 mm scope kit complete with a backpack to carry it in
http://store.meade.com/telescopes/et...-backpack.html ), but I'm not sure you'll have much luck finding a 6-incher that would go in a backpack.