I was going to opine 35 liters, off the cuff. However, our resident packing / volume expert "DougFitz" has weighed in. Seriously, he KNOWS his stuff. Doug has worked this all out. Take it to the bank.
This also explains why I keep coming back to my 48 liter Osprey Kestrel rucksack. I have tried smaller capacity bags, only to prove each of Doug's data points each and every time. Compression (kg / liter) and the overall mass (weight) of your stuff yields the optimum rucksack size.
After four Caminos, and testing packs from 30 to 48 liters I have reached these conclusions:
I must carry about 12 Kg, for reasons of health, diet; and being a large fellow, my stuff is larger and weighs more. Try as I might, I have only once gotten the weight down to 11 Kg, all-in.
Using Doug's rules-of-thumb (above) 12 Kg at about 200 gm / liter equals about 44 - 45 liters. My 48 liter pack seems to work well under all conditions, in all seasons. It also affords me enough capacity to stash everything inside the pack, without any "dangly bits" on the outside. I do not like "dangly bits." They are IMHO unsightly, make noise while you are walking, and are susceptible to being "appropriated" by others at rest stops.
Finally, my only opinion is to allow extra space for things you pick up along the way, even if it is only snacks, or the occasional souvenir. In my experience, on a long Camino, extra items seem to "sneak" into your rucksack...
Bottom line...listen to Doug...he's real smart!
I hope this helps.