I made my first Camino, a part of the French Way, last September. I bought a 38 l Deuter backbag
and that was just enough for me. 35-38 l is nearly the same. No need to carry extra clothes or things.
Most important was how easy it was in use- enough pockets to keep water, maps and money handy.
And it must be good for your shoulders and hip. Test it carefully. I didn´t like too many straps.
And of course rain cover.
Buen Camino! Tuulis