I am 5' 1" (155 cm) tall and weigh about 115 lbs with a very short torso. I'm looking at a Gregory Women's backpack in XS which is 56 litres. Does this sound reasonable to hike the Camino Portugues? I have read that people like the Osprey backpacks on here, but they don't come any smaller than Small and according to another article I read, with my short torso, I would need an XS. I know I need to go and try them on, but just looking for a wee bit of guidance. This will be my first Camino, but I've hiked with a friend in Wales (Pembrokeshire Coastal Trail) and Ireland (Wicklow Way & Coastal Trail) and most recently, Les Sentiers de L'Estrie in the Eastern Townships of Quebec which was the most gruelling hike ever. We just did day hikes and stayed overnight on all these hikes. This time we are planning to do from Porto to Santiago, averaging 20 km a day.
Brenda
Forget the cargo capacity of the pack, ie the number of liters it will hold. That is not the important measurement.
First, get an accurate measure of your torso, your waist, and determine what shape the shoulder harness will best fit you. Once these measurements are dialed in, then you can determine what size pack -- usually expressed as "small", "medium", "large", "extra large", although these delineations in size can be combined, depending on the manufacturer (S/M, M/L, L/XL).
As others have stated, the pack must be tried out in person to decide if it will work for you. The less experience one has with wearing a pack, the more time it takes to be reasonably certain it will properly handle the load and will be reasonably comfortable while carrying it for long hours at a time. And little things about a pack can mean the difference between having a peaceful coexistence with your pack VS having the pack be so irritating that you're ready to fling it off the next cliff. Be prepared to load the pack in the store, or at home, with around 20 to 25 pounds weight... a proper store will have bags of sand or pebbles to do this --- and spend a good thirty minutes walking around the store, up and down stairs, wandering around to see how the pack feels. Likely, you will be carrying less weight than 20 or 25 pounds; but you are needing the heavier weight to help compensate for a limited "try out" period in the store.
For example, how much does it matter about having your back perpetually soaked in sweat while hiking with a backpack? Some pack manufacturers do a better job of back ventilation than others. Can you live with minimal side and back outside pockets, or do you want to avoid having to dig into your pack to grab frequently needed items? Does the hipbelt properly mold to your hips, or does it just grab them causing chaffing and irritation to the skin?
Lot's of stuff to consider about backpack choices, and though you want to check out models that seem "popular", there are a lot of manufacturers that are well worth considering. In addition to Osprey (which I personally dislike for anything other than a day pack), ULA, Gossamer Gear, Zpacks, Deuter, Mountain Laurel Designs, etc... all offer terrific packs.
Here are a couple of links to some pack fitting and sizing sites. Ignore any brand affiliation as the information is pretty pertinent regardless of brand.
https://www.rei.com/learn/expert-advice/backpacks-adjusting-fit.html
http://blog.gossamergear.com/how-to-size-and-fit-an-ultralight-backpack
https://sectionhiker.com/how-to-fit-a-backpack/