Everybody is built differently, so there is no backpack that will fit everybody perfectly. For example, in the old days, some backpacks had an adjustment for shoulder width. In other words, the places where the shoulder straps were attached to the main bag might be wider or narrower. Currently, about the only way you get there with a modern backpack is simply by backpack size, even though that mostly pertains to the length of the torso that it is intended to fit. In the old days, I could wear a large backpack, but the shoulder straps would be too far apart due to my narrower shoulders. There were some things that I could do to the strap tops, but mostly I had to "snug" the shoulder straps closer together in the front via the sternum strap. When using the backpack for skiing, I needed to keep the sternum strap snug for balance. If I had wide shoulders, then a lot of this would have been unnecessary.
At the hips, everybody is different still. Some people have no hips that are visible. So, if you have hips to allow the waist belt to rest down onto, that is one way to wear it. If you have no hips, then you might have to simply squeeze the waist belt around you. Either that or put on five pounds of flab on the hips.
In general, a skier backpack needs to be very snug against the back. Often there is some space there for an ordinary summer backpack.