Your sleeping bag goes down to 20" x 10" and would fit in a backpack fine, if you had the right one for your gear. If you don't have a pack, you might take your gear to the outfitter/store and find one that fits your current gear. Renting packs until you find one that works for you (or until you're done tweaking your gear list) might also be do-able depending on your budget. At the very least, I'd take the bag to REI and have myself measured for right pack size, pick a pack, stuff in the sleeping bag then a bunch of REI sandbags, and walk around the store for a while until you find the pack that works best for you *and* works with the bag.

If you are anything like the majority of folk who start backpacking and keep doing it, however, you may find yourself with a closet of gear anyway. <img src="/forums/images/graemlins/smirk.gif" alt="" /> I do my best to keep my pack small and light, so put my 3 lb synthetic doesn't-compress-well bag in the closet and switched to a down quilt that packs to the size of a football.