It is determined by measuring your curving torso length from the 7 th cervical vertebrae down to the horizontal line between the iliac crests. I know this may be confusing to understand. So stand straight up and look down. Now draw an imaginary line between your left and right iliac crests. Where this line intersects with your spine will give you the second point in the measurement.
Additional pockets are able to be added on to the tension-straps.
However, the pack is large enough to accommodate most peoples gear internally for extended trips. In addition, the tension-straps are one yard longer than the pack and can hold almost anything.