The ideal size for your travel backpack will depend on where and how you plan to travel. If you plan to use it as a suitcase to carry clothing and shoes on planes, you’ll want the largest possible bag that meets your airline’s carry-on bag guidelines. That’s usually somewhere around 14 x 9 x 22 inches (WDH) for bags in the overhead bins and 13 x 9 x 17 inches (WDH) for a personal item (though most airlines never enforce the latter). If you’re using your bag for daily commuting, you’ll want something more slim and narrow, especially if you take public transportation. For most people, a 13- or 16-liter bag is large enough to carry a laptop plus daily use items like a water bottle, jacket, keys, chargers, and more.