HomeFor a hostelling trip, a minimum of 50 litres is needed, depending on how long you are travelling and whether you'll be taking your own sleeping bag or other items. But for camping and hiking, you'll need at least 60 or 70 litres. For an intense expedition, you might even need to consider 90-100 litres or even bigger.
Be careful not to get a bag that is too big though. If the contents can move around easily, the bag is likely to get uncomfortable. And if there is lots of space, you may be tempted to fill it with heavy souvenirs.
You also need to take the material strength into consideration.
Small bags may not need to be very strong, but be aware that school bags are likely to be thrown around by unruly children so take the usage into consideration, not just the size. Obviously, larger bags will need to be stronger to carry the extra weight, and outdoor expeditions will also put extra strains on your bags so this will dictate even stronger materials.
The strength is given by the material thickness. 300D (denier) or less is fairly standard for small bags (but consider investing in a stronger one if the needs dictate this). For hostelling and travelling, you should be looking at 600D or more, especially for camping trips. Intensive outdoor expeditions should really be using bags with 900-1200D.
If the bag will be used outside a lot, it may be worth considering a waterproof rucksack, or a waterproof cover for it. Remember, though, that water will still be able to enter via the zips and openings, so these are not suitable for full submersion. Dedicated drybags will be needed if you are likely to be dropping your bags in any water.
Some rucksacks also have external webbing which allows your to attach other items to the outside of your rucksack. This can be useful for carry tents and other large items, enabling you to buy a cheaper, smaller rucksack and still be able to carry the same amount. It is also a useful back up if you run out of space inside.
A large bag with a single space can make it very difficult to find your smaller items. So a lot of larger rucksacks have side pockets and internal separators so that you can divide your possessions into suitable compartments. This makes it much easier to find your smaller items (such as your passport when you're being hassled by an impatient border guard).
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