Whatever size pack you ultimately decide on, PLEASE, for the sake of your Camino have it properly fitted in a shop that has experienced hikers on staff.
Your footwear and rucksack are the two most important pieces of kit / gear you will obtain. Everything else is just filler (things stuffed inside) and "dangly bits..." (things hung on the outside of you or the rucksack). Please DO NOT Scrimp on these two items.
I recommend filling the considered rucksack(s) with perhaps 10 kg (22 lbs) dead weight, like sandbags or jugs of water (I use sacks of kitty litter), and wearing that pack in the shop as you shop for other gear, even footwear. Really good outdoor shops will have weights like this for this purpose. They will also have a plastic fitting measuring frame to assess your torso to recommend the size frame you need.
After 20 minutes or so, you will have a good idea of the fit. The properly fitted rucksack feels like an extension of you. In fact, at the end of a six-hour walking day on Camino, when you take it off, if you feel figuratively "naked," the pack was likely fitted correctly. If you "miss it" when it is not worn full, it is fitted correctly. Pain is not necessary...
Rucksack weight and volume capacity are only some considerations. The fit and features the particular bag has are also key to your satisfaction, especially if you plan to live out of this bag for a month or more. Invest in the rucksack that will hold only what you will actually pack for your Camino, not for some future possibility. Overly large volume bags tend to attract stuff along the way...
As regards size, as someone stated above, torso length / height is critical. Most better branded rucksacks come in short, medium (S/M) and large (M/L) torso sizes. This in turn affects the volume. But more importantly, it affects the comfort possible when fine-tuning the various straps, harnesses and belts that this better bags come with. Also, different manufacturers have different size ranges for their S/M & M/L bags, so Caveat Emptor...read the fine print...
Also, capacity varies with frame size. For example, most Osprey brand rucksacks are available in a S/M & M/L torso length internal frame. My previous Osprey Kestrel 48 liter bag in M/L was a 48 liter capacity bag. However the Osprey Kestrel 48 in the S/M torso frame is actually only a 46 liter capacity rucksack, as a result of the torso frame shortening the main compartment a couple of inches / several cm.
After five Caminos, and a family trait of spinal stenosis, I my spine shrank (compressed) some 5 cm from my first Camino, 183 cm height in 2013. My physical examination in December 2016 indicated that I had shrank a total of 5 cm (@ 1.5 inches) from 5 years before. As a result., I was unable to find a good, comfortable adjustment on my Osprey Kestrel 48 liter bag that would prevent the shoulder and back pain at the end of each day. After five years, my bag outgrew me...
So, needs must, I made field-expedient shoulder pads from sponges wrapped in microfiber "dish towels" purchase in a supermercado. The towels were fastened in place with duct tape. The homemade shoulder pads were then mounted on my shoulder harness to raise the strap some 5 cm from my shoulders when compressed...that took six sponges, three to a side...
When I arrived at Santiago, I donated my used but in very good condition rucksack to the Pilgrim House on Rua Nova 19. On return home, I immediately purchased the identical bag in the next smallest size, an Osprey Kestrel 38 liter bag, but in a S/M torso frame. This meant that the actual volume was 36 liters. I knew this when I ordered it.
The end of that saga is that, loaded with said bags of kitty litter to about 20 pounds / 9.5 kg +/- it fit me perfectly after requisite adjustments. I have been on several shorter day-hikes with the new bag loaded with dead weight. I am looking forward to my first real Camino with it.
You might wonder why I obtained a 10 liter smaller rucksack. The answer is simple. After five Caminos I have finally perfected my personal packing list. Over the years, I sent too many boxes of "surplus to needs" stuff to Ivar to hold for my arrival at Santiago - a VERY good service BTW.
Also, on my last two Caminos, I discovered that the 48-liter bag was not full by the time I reached the final week or so towards Santiago, and had no dangly bits. In plain English, this meant that 48 liters was too large for me in the first place.
But, like many of you reading this, I was a "newbie." After five Caminos, I finally figured out what really works FOR ME.
So, my suggestion is that, depending on personal needs, packing style & preferences, together with the torso-length factor, as well as the target 10-kilogram / 22 pound suggested upper weight limit, something in the 32/34 to 46/48 liter capacity range should do for perhaps 95% of all pilgrims. In the end the answer is, it depends...
I hope this helps.
P.S. I recommend highly that you consult 'Dougfitz' postings on rucksack volume vs. packing weight. I regard him as the forum technical expert. He has actually figured this stuff out using numbers! Crikey! You can use the search function in the upper right hand corner to search for postings by "Dougfitz."