These are good sized travelling rucksacks. If you are hostelling or hotelling, this size should provide you with all of the space that you need for all of your belongings and a few souvenirs too.
This size is also ideal for camping, where your tent and other large items can usually be attached to the outside of the bag, leaving plenty of room inside for your other possessions.
With the amount of weight that you can carry with this size of bag, look for additional comfort features, and make sure that you have a waist strap if you'll be carrying it for any length of time. A Material thickness of 600D is an absolute minimum, but look for 800D or higher if you want it to last longer than a few trips being thrown about.
You may also need to consider the shape. Some bags are short and wide, others are tall and thin. If you will be cycling, you will need to be able to see all around you, so make sure you get a bag with good all-round visability.
A single large compartment on this size of bag is asking for items to get lost, so look for separate compartments and additional pockets which will allow you to segregate the contents so that you can find them easily.
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