okay but why?
it's far far far more important to have a well-fitting backpack
after a few days you won't even notice it's there
if you're not an experienced backpacker go to a store like rei that specializes in outfitting hikers
and look everybody will laugh and point and think you're dorky but the truth remains that external frame backpacks transfer more of the weight of the load to your hips than internal frame backpacks (this is good, this is what you want) and they stand away from your back so you'll stay cooler and drier