Stores like REI should have a torso measuring device that works much like a Brannock Device-the gizmo you use to measure your feet. it has a hole that fits over the bone at the base of your neck, then a sliding plate that comes down to your hips. It is made of plastic, as I recall or maybe stiff cardboard.

Get a pack that is adjustable. Some of the less expensive or lighter ones aren't. The more adjustments you can make, the better the fit unless you just happen to be the right size and shape.