Select the Right Backpack: Torso and Hip Size
The right backpack will not only fit all of your gear, but it will also fit you. It may seem odd, but hiking and camping backpacks come in different sizes. Like wearing the wrong size underpants, the wrong size backpack will make you miserable fast.
You may think that your height and girth dictates the backpack size you need, but that's not exactly right. Your torso length and hip size, not your size overall, are what you need to know to find a backpack that fits well.
To find how long your torso is and what size your hips are, get a flexible tape measure (not the big yellow Stanley one from your toolbox -- but rather, a sewing tape measure). If you don't have one, you can use a long piece of string or ribbon. Then get a friend, because unless you're an extremely talented contortionist, you just can't do this alone.
Tilt your neck down and have your friend locate you seventh cervical vertebrae. When you're looking down, it will be a hard bump at the base of your neck, where it meets your shoulders. Have your friend take the tape measure from this vertebrae to your iliac crest. Your iliac crest is the top of your pelvic bones. It's easy to find -- run your hands down your ribs. Once you pass your ribs, you'll come to your soft belly (maybe you should do some sit ups or order a salad once in a while). The next bit of bone you hit is your iliac crest. Have your friend measure the distance between your seventh cervical vertebrae and iliac crest (if you're using string or ribbon, mark the distance on the string, then measure that). That's your torso length.
To measure your hips, take your tape measure or string and wrap it around the top of your iliac crest, right where the tops of the bones of your hips stick out. That's your hip size.
Use these measurements when checking specs of the backpacks you're interested in. Most camping and hiking outfitters list the range of torso and hip sizes each backpack they sell will fit, as well as what ranges of each measurements different sizes of the same packs will fit. Buy a pack that fits.