What Nick said is certainly important, but there is another option: get a pack without a hipbelt, then the size of the pack won't matter. Of course this presupposes an UL kit. My summer pack, a Gossamer Gear Whisper which by design is beltless, usually weighs in at around 11-13 lbs total (food , water, shelter etc.). My winter pack, a Golight Jam, is often carried without the belt clicked, something I can do because the total weight is under 20 lbs. However, if you're carrying more weight than that you'll definitely want a hipbelt for better load distribution. Happy trails!
McHale Packs here in Seattle do custom jobs, check "em out!