If you’re looking for a child's backpack, fifteen inches is a standard size, which increases to 19 inches for an adult. I’d always look for a backpack 19 inches and larger to ensure it fits everything I need in without being too much of a squeeze. Unfortunately,  I’ve been caught too often when I’ve bought a backpack online and realized that my belongings don’t fit once it arrives, so I always thoroughly check the dimensions before making a purchase. In terms of capacity, 30-50 liters is reasonably standard.