@HoughtonLikeTheLake
It's a really smart move to get this part of your trip dialed in before you head out; proper backpack fit can make or break a trip! @REI-JenK is spot on with the recommendation of a virtual outfitting appointment for talking through the specifics of fitting a pack to you. That's the best way to get the specific advice you need without having to make the trip in to a store. To get you started here though, check out this Expert Advice article on How to Size and Fit a Backpack, paying close attention to the video and section about torso length.
The next step is to adjust the pack to what your torso measurement is, which sounds like should be close to 17". The shoulder harness on your pack is secured to the pack by a hook and loop (velcro-like) attachment just under the end of the shoulder straps by the body of the backpack. All you have to do is separate the two and you'll be able to move the shoulder harness up and down (pro-tip: a small, thin cutting board or plastic sheet can be helpful for separating these two pieces and save your cuticles from scraping against the hook and loop fabric).
The Gregory Maven has marks on the harness indicating the size. It appears as though the marks start with 'S', then there is another mark, and then an 'M', and likely another mark. You can check with a tape measure to see how far the marks are from one another, but they are probably about an inch. That would mean the 'S' is about 16", the next mark is about 17", etc. After setting the harness at your approximate torso length, try the pack on and look in the mirror. Pay close attention to the fit of the harness, hipbelt, and the angle of the load stabilizer straps as talked about in that Expert Advice article.
If the fit looks and feels good, put some weight (5-10 pounds) and some fill (pillows or towels) into the pack and put it on again. Repeat the process in front of a mirror (this is where the virtual outfitting appointment would be useful as the outfitter can look for fit issues) and check the fit again.
If everything looks and feels good at that point, we recommend loading your pack (with a similar load that you expect to carry on the trail) and spend some time walking with it on. Pay attention to how it feels and moves with you and don't hesitate to make some minor adjustments to really dial it in.
Hopefully this helps, be sure to come back to the community and share some pictures from your adventure!
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