Backpacks should fit your back comfortably and feel comfortable, not too bulky or poky. However, when it comes to backpack size, that depends entirely on the way you plan to use it. For example, if you are looking for a backpack or hiking bag, you will want to make sure the hip belt fits directly on your hip bones.
Or just underneath your hip bones if you have slim hips. Furthermore, your shoulder straps should begin at the curve of your back (not too low down your back). Before trying on packs, it’s a good idea to measure your torso length to ensure that they fit you perfectly.