One of the first rules of style is fit. Nothing is worse than a product that doesn't fit your size, style or needs. Bags, wallets, or luggage are no different. In order to know your size though, you need to be measured.
The first step to finding the correct size is finding your own torso length. This measurement will give a a good idea of weather a bag is going to be too large or too small for your frame.
Torso length is found by measuring from that bony bump on the back of your neck (your C7 vertebra) down to your hip bones (iliac crest). You may need help from a fellow traveler to get this measurement.
This torso is 17 inches
Once you have this measurement keep it in mind when looking at the height of our bags. Anything larger is going to cover more of your back and be harder for your frame to support while anything smaller is generally going to fit smaller in other areas as well.
You will also want to measure your torso width. To do this measure shoulder blade to shoulder blade.
This torso is 11 inches wide
Generally the other considerations are how much stuff are you wanting or willing to carry? In our experience any empty space is quickly filled so if you wish to carry less, buy a smaller bag.
Our bags space is measured in liters. This measurement is found by filling the bags with packing beans or similar materiel and then measuring the volume of the materiel. This accounts for the space and stretch of bags far better than simply measuring the length, width, and depth since our bags are not rigid cubes.