To tell the truth, most people don't need a bag that is more than 100 liter. And in theory, the weight of each person's back should be only one-third of his own weight at most. There are too many things to pack, and only you know how difficult it is to recite them. So, relax.
When buying a bag, you should not only look at the capacity and quantity, but also look at the size of the suspension system you should use. Before choosing a bag, you'd better ask your family and friends to help you measure your torsolength with a ruler (preferably a tailor's soft tape measure). This trunk length refers to the distance from your seventh vertebra (the most prominent bone at the junction of neck and shoulder) to the vertebra parallel to your hip bone. The length of this trunk meets the needs of your inner frame. Don't think you have to carry a big bag of 1.8 meters. Some people have short legs, while others have short legs.
Generally speaking, if your torso is less than 45 cm long, you should buy a small bag. If your trunk is 45-52 cm long, you should choose a medium-sized bag. If your torso is more than 52 cm long, you should choose a big bag.
Good manufacturers provide oversized (more than 56 cm) or oversized (less than 40 cm) bags, which can also be tailored. The bag sizes of major manufacturers are different, even the bag sizes of different series of the same manufacturer are different. You'd better ask clearly or find some information. If not, measure the length of the inner frame.