If you are a hiker, one thing to get right is your backpack.
Size of Backpack
You need to consider the right size for you. One way to figure this out is to measure the vertical distance from your neck (from the bony bump at the base of your neck to your the center of your back at the hip bones). You may need a helper to do this.
16″ or less, get a size Small
16.5 – 19.5″ get a size Medium
20″ and more, get a Large