To confuse and bemuse, I am here to testify that others measure you wrong a lot. I had to follow McHale's instructions for self measurement and do it a few times to verify, to find out that all those trips to the local outfitter and to REI, the staff failed utterly at getting within an inch of my actual back measurement. This made a huge difference for me since I am right in between the small and regular/medium sizing for most pack brands. Three backpacks of the wrong size later, I figured it out.

Now I ignore the labels "small" and "medium" and go straight to the website of the pack under consideration to look up sizing in inches, and match to my actual back measurement - which is what I suggest. Also, you will want your hip measurement, as packs with fixed, nonswappable, nonadjustable hip belts don't work for everyone. For me, this means any Osprey "ultralight" backpack - aside from the frame just being wrong no matter the size because they make me sore, I can't get the hipbelt swapped so can't use the pack.

So the steps I suggest to you to save yourself time and money:
Google McHale backpacks and read the sizing instructions.
Measure your back and hip sizing - ask a spousal unit or good friend to work with you to verify if you want.
Go directly to the website of the packs you think you might want and look up size ranges.

McHale makes awesome backpacks, by the way, and makes them to order, specific to your sizing.

The packs I have ordered online have tended to be the ones that work for me best. Granite Gear has a great policy - if you buy one (from anywhere) and find the hip belt or shoulder straps don't work for you, you can ship it to them and they'll swap them out for you for free. The Nimbus series has an adjustable frame, swappable straps and belt, and hauls quite a load.

The pack should also be able to carry the bulk and weight of your gear - generally, a 55-60 liter pack will get you started if you're renting a bunch of stuff from REI. My usual 3-5 day backpack is a 40 liter. I got rid of the 5 lb tent, the 3 lb sleeping bag, the 2 lb sleeping pad, and bulky fleece layers, and also the extra doodads I didn't need and didn't use.