One of my most frequently asked questions about backpacks is about the size. Everyone wants to know what the perfect size is. No one backpack size is better than another. What matters is that your backpack should be proportional to your body — that might mean a backpack that is 40 liters or 60 liters. If your backpack is too big or too small, the weight won’t be balanced properly and will cause back pain or maybe even make you topple over. You don’t want a skyscraper rising up from your back, but you also don’t want a pack that is clearly too small and overflowing with your stuff.
You want a backpack that is big enough to hold just a bit more than the stuff you are bringing and not more than that. If a backpack fits everything you want, has a bit of extra room, and feels comfortable, then you have found the perfect backpack size. Manufacturers also have suggested torso and waist sizes for each model they produce, but I’ve found that the best way to know if a backpack feels right is to simply try it on.
When you are at the store (and any good camping/outdoors store will do this), they should be able to stuff your backpack with the equivalent of 30 pounds (15 kilograms) so you can see how that much weight feels on your back.
It’s important to remember that the bigger your backpack is, the less likely it is that you’ll be able to carry it on the airplane. Additionally, since you can no longer bring liquids in containers larger than three ounces on airplanes if your bag has soap and liquids in it, you’ll be forced to check the bag. Most baggage sizes are 45 linear inches (22 x 14 x 9 in) or 115 centimeters (56 x 36 x 23 cm) including handles and wheels so if you get a backpack with those dimensions, you’ll be able to carry on.
You won’t face any baggage fees from the major airlines for checking your bag when flying internationally. Budget airlines, on the other hand, charge a fee for checking a bag based on weight, so the more your bag weighs, the more you will have to pay to check it at the gate. Even though my bag fits in the overhead bin, I often have to check it when flying a budget airline.