To find your backpack size, measure the length of your torso and hip. To do this, have a friend help you measure from the bony protrusion at the base of your neck (your C7 vertebrae) to the top of your hipbone. Alternatively, you can measure yourself by holding a tape measure at your C7 vertebrae and running it down to the top of your hipbone. Once you have this measurement, consult a size chart to find your backpack size.