What is a good backpack size for the Camino de Santiago?

Camino de Santiago Backpack
Camino de Santiago Backpack

The question was:

Since I’ve decided that I will alternate between albergues/refugios and hotels, I’ll now need to pack a sleeping bag as well, right? Therefore, if I consider minimal clothing, a tiny first aid kit, food, water, and a sleeping bag, how many cubic inches should I be looking at?

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