An editorial question about judging a loaded backpack rather than relying on its size label.
1 answer
Check it with a representative load, then watch what happens when you move. A useful fit assessment combines where the harness sits with whether the bag stays comfortable during ordinary walking.
Osprey’s guide says the shoulder harness should wrap around the shoulders without gaps, and a load-bearing hipbelt should be positioned over the hip bones. Its sizing approach uses torso length because two people of the same height can need different pack sizes. If the harness will not line up while the belt is in the intended place, revisit the size or torso adjustment.
You can make the rest of the trial quite practical. Put on your usual travel layer, turn your head, reach for a pocket and walk a short route. Notice whether one strap slides outward, a rigid item presses through the back panel or the load shifts every time you turn.
Those observations also help separate fit from packing. A hard charger against your back may need a different pocket; a harness that is fundamentally too long needs a different adjustment or bag. Take notes about the specific problem before comparing another model, so the next try-on answers something concrete.