>>2277173>is 40 liters enough for 3 nights outside?Depends a lot on what you're carrying.
For example, I just tried with my old supermarket gear (as in a 10€ 15ºC sleeping bag and a 15€ tent I got at the supermarket years ago for my first time camping), can get both inside my equally old 25-30L backpack from LIDL; tent poles go outside, bottom in a bottle pocket (if the pack has them) and held firmly with the compression strap, the tent "tube" can be folded in half and above the tent and sleeping bag there's still space for six 1.5L bottles upright. (picrel just to give you an idea)
If you have reasonably decent gear (smaller tent, a sleeping bag that compresses well, etc.) you would have even more free space
But if anything, I'd rather err by excess (i.e. I'd rather get a pack that is slightly too big than one that is slightly too small).
I would go with 50-65L until I'm really sure about what I have to take and how much space all the stuff takes. (60-65L would be a really safe bet)
