If you're looking for a weekend hiking pack, you'll probably want something around 45L in size, depending on how much you need to carry. To accommodate a lightweight tent and sleeping bag, food, and cooking equipment, it's better to err on the side of caution and go for something around 55 to 60L. After all, if your weekend hike goes well you might want to think about tackling a longer route next time. For that you'll need a full-sized pack. Expedition backpacks can be anywhere up to 80L in size or even more, but carrying a fully loaded bag of that size is quite a challenge even over easy terrain.