Based on three Caminos: French (2013, 2014) Portuguese (2015), I recommend the smallest rucksack or backpack consistent with the bare minimum amount of stuff you intend to carry. In this regard, consider VERY CAREFULLY everything my colleague "DougFitz" has to offer to us all. Of all the veterans out there, I believe he is the only person to have actually analyzed all the aspects of what and how you pack stuff on a Camino. He has it down to a series of well-developed mathematical formulas. Really, check his postings out.
This said, I have used my Osprey Kestrel 48 liter pack for all my Caminos. I am a large fellow, 5'11" 225 pounds (183 cm, 102 Kg) so all my clothing items are necessarily larger and heavier on an item-by-item basis. This is something seldom considered in making comparisons. When considering advice from this forum, or from any other source, always put the information into the correct context. Compare large persons to large persons and smaller built persons to like sized persons. Do do otherwise could lead to incorrect results.
On completing this third Camino on 10 April, I had a number of take always or "light bulb moments." These are my periodic flashes of "DUH! Now why did I not think of that BEFORE thoughts and reflection." Many of these flashes of brilliance have been made here REPEATEDLY by many pilgrims far more experienced than me. But, here are my flashes relative to pack size only:
1. Allow your pack load volume to determine your pack size, not the reverse. Most folks, including me, have a particular rucksack they like and proceed to fill it so it looks like a proper rucksack. However, by that time you have no doubt packed too much stuff. One size DOES NOT fit all Camino routes or time-of-year requirements. Walking the Camino IS NOT the same as walking across the university campus, or commuting to work on public transit.
2. Wear one complete outfit and pack only one replacement, except for socks and perhaps undergarments. Pack three of those. In fact, consider going "commando" and make do with one pair of zip off synthetic trousers, and only one other shirt, undergarment and sock change. Based on direct experience, I would opt for a third set of shirt and underthings ONLY if you reasonably expect cold and wet conditions. I am NOT referring to the daily Galician rain showers, fog and mist. I AM referring to the days and weeks of continuous wet weather one can encounter, especially on the Frances in April, May, October and November. I have encountered snow at elevations along this route during mid-May in both 2013 and 2014. However, if walking usually drier routes, like the Porto to Santiago stretch, do not take too much.
3. If in doubt, LEAVE IT OUT! Enough said on that point. It rather speaks for itself.
4. Be brutal in cutting weight...see recommendation #3...repeat as necessary. I REALLY, REALLY like Ivar a lot. However, my per Camino costs for his storing items I ship down the road to him is starting to be embarrassing. I, of all people, should know better after three complete Caminos. I suppose I am trying to live up to Albert Einstein's definition of insanity. Some decades ago, in a speech he was quoted as stating that: "(insanity) was doing the same thing over and over again and expecting different results..." Well, everyone needs a goal in life...
5. I have come to the PERSONAL CONCLUSION, based on my experiences, expectation, Camino pace and style, that the lightest, best fitting rucksack of about 35 liters is a good target size, FOR ME. DougFitz may take issue with this conclusion, and I respect his opinion. However, you need to also consider that I am unable to stay in albergues; (a) I snore and am being considerate, and (b) I catch respiratory crud very easily. So, I no longer carry a sleeping bag, and I stay in hostels. However, I have recently found a very lightweight, rectangular, microfiber sleeping bag liner, with a full zipper...but I digress...I suppose it could do double duty as a uber large bath towel in a pinch.
Last year I bought such a lightweight pack online in France. It is made of ripstop nylon, has an internal aluminum frame and weighs just UNDER one-kilo empty. That is ONE-HALF the empty weight of my beloved Osprey Kestral 48. It is made by a firm called Elementerre. Their web site is:
www.elementerre.fr. Click on "Products/Bags/Arenal 35" to see the pack I bought. However, I have NOT YET USED THIS RUCKSACK on Camino yet. I hope to do so later this summer on the Camino Ingles with "BiarritzDon" and friends. However, as soon as I am medically cleared to do so, I have this rucksack loaded with a 15-pound bag of kitty litter (in a bin bag to contain dust) for training walks. My personal goal is to meet or surpass (downward) the much-vaunted 10 kg rule.
6. Bottom line, IMHO less is more when walking the Camino. You don't NEED whatever "just-in-case" item you need. And in the statistically unlikely chance a need did develop, you could either obtain the needed item from a local shop, donation box at an albergue, or from another pilgrim.
I sincerely hope this helps.