The size of your bag will depend on your needs. If you’re planning a month-long trip to Europe you will want a size that will accommodate all your essentials. A daypack can suffice if you’re looking for a quick trip or if you’re looking for something that’s adequate to hold all your camera equipment. Once you decide what you’re going to use your bag for, you will want to be sure it fits your torso properly so be sure and refer to an Osprey sizing chart.