Welcome!

You may want to read this useful article on backpack fitting on the home page of this site. Note that the important measure is your torso length, described in the article, not your height or weight. The size (small, medium, large, whatever) is liable to be a different torso length for each pack maker, and sometimes even for different models from the same maker.

If your backpack doesn't fit properly, you can be in for a lot of misery, so please do whatever it takes to find a good fit. All of you, from your shoulders to your toes, will thank you!
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey