The L/XL back length is designed for torsos measuring over 50 cm from the base of your neck (the C7 vertebra—that bony bump) down to the top of your hip bones. If you're tall or just have a long torso relative to your height, this size is essential so the hip belt lands correctly and actually carries the weight instead of your shoulders doing all the work.
Don't go by height alone—some tall people have short torsos and long legs, while some shorter riders have surprisingly long torsos. The only reliable way to know is to measure. Have a friend help you find that neck bump and measure straight down to your iliac crest (the top ridge of your hip bones).
If you measure over 50 cm, the L/XL is your best bet. A pack that's too short will ride awkwardly, the hip belt won't engage, and your shoulders will be screaming by lunchtime. If you're right on the border, look for a pack with an adjustable back length so you can dial in the perfect fit.