The point about the load lifter angle is an important one, too. It does affect pack length, since it has to attach high enough to make that angle. The usual recommended angle is 45 degrees, but that’s not absolute. Some packs (especially for loads under 30 pounds) don’t demand a 45 degree angle; some (including Deuter) actually say the load lifters aren’t for load carrying, but just to keep it from pulling backward - they recommend an angle of 10-30 degrees (if memory serves.) Just as important is where the lifter attaches to the shoulder strap; it should be just in front the top of the shoulder straps, usually. Again, Deuter allows you to adjust this contact point, also. (As I think of it, you might want to take a look at Deuter’s adjustable packs; you might find a good fit there. I’ve used the ACT Zero 50+15, and found it very good when my loads were in the 22-27 pound range. Right now, with a load around 18 pounds, I prefer the Osprey Exos 48 which weighs a pound less.)

Good luck!