To get a backpack that fits you perfectly you need to focus on your torso length and not your height. Here's a general guide for what size backpack you should get based on your torso length: ES: 15 inches S: 16-17 inches M: 18-19 inches L: 20+ inches Another factor to look at when determining your backpack size is the waist belt. It's important waist belts can be adjusted to cinch your waist because they aid in supporting your load's weight.