Can you bring a backpack into the festival?

I see on the FAQs that you can bring a bag <10L. Would a typical sized backpack be acceptable? Or does it need to be smaller? I'm just not certain of the measurements of my bags so want to get an idea ahead of time. Also, if someone brought a bag that was too big. Can they put the bag in a locker and still access it, or is it not permitted in the festival at all? I'm not camping, I'm staying in Nantes.