What is the ideal size of the backpack for the Camino de Santiago?

What to bring with you on your Camino de Santiago
Backpack size on the Camino de Santiago

The question was:

I plan to go shopping this weekend but am unsure of what SIZE a backpack to purchase?

Read what our veteran pilgrims say about a good backpack size for the Camino de Santiago.

Also have a look at this list on the topic of backpack sizes for the Camino de Santiago.