When deciding what size backpack to take on a three-month long trip, consider the type of traveller you are. If you plan on doing a lot of hiking and camping, you'll need a larger pack that can accommodate all of your gear. But if you're more of a city-dweller, a smaller backpack will do just fine.   Another important factor to consider is the climate you'll be travelling in. If you're headed to a tropical destination, you won't need as many clothes as someone who's going to Europe in the middle of winter. Keep this in mind when packing your bag so that you don't end up bringing more than you need. Finally, think about how much stuff you really need to bring with you on your trip.