Right now we go day-hiking & I carry the few things we bring in my Army-issue assault pack...

Recommend an internal frame pack.

Take wife unit to the nearest REI store.  Bring a new bag of dog food (weight simulant) and try a bunch of packs out weighted down.  

5-day trip being your ultimate goal, buy a slightly larger pack than you think you need.  Just be careful not to fall into the trap that your bag must be fully packed when you go on a trip.  The more room you have the more crap you'll pack.  Leave the crap at home.

Yup.  Don't buy it online unless she's tried it on in the store.  And have her try on some hiking boots while you're there.

Osprey packs. Buy once, cry once.
Lifetime guarantee and they can custom mold the hipbelt to fit her perfectly.
You can usually find them at REI or Backwoods.

For day hiking (if you need a pack for this) a REI Flash 22 would be fine.

For 3-5 days, look for an Osprey pack [female] in the 50-60 liter area.

Female Gregory packs are good as well, but they tend to be heavier.

Second on the osprey packs!!! i have a Osprey that i use for 3-5 day trips. Internal frame and very comfortable to carry for long periods. I also have a Mystery Ranch that is my 1-2 day pack that is my favorite pack! If you can got to REI or a similar outdoor store and try some packs on. Kelty, Deuter and Gregory make good packs as well! I brought both packs on a 3 day camping and climbing trip this summer. I used the Osprey to hold all my camping gear and i had my Mystery Ranch packed with climbing gear and all the rest of the gear i would need in the canyon climbing all day.

http://www.amazon.com/Kelty-Womens-Coyote-4500-Backpack/dp/B001M0MIO8/ref=sr_1_22?s=sporting-goods&srs=2594700011&ie=UTF8&qid=1417471686&sr=1-22&keywords=kelty+womens+backpack

This is what my wife carries on our BWCA trips.  She likes it very much.  Might be a little larger than you need but you don't have to fill it and can cinch it down if not full to balance the load.  She vastly prefers it to the gear she was issued, as in not even a comparison.

I like Kelty packs.  They've given us years of service for relatively good prices.  I have a smaller one for shorter excursions that isn't hers but she does like it as well.  http://www.amazon.com/Kelty-Redwing-50-Liter-Backpack-Forest/dp/B009R6B3NQ/ref=sr_1_1?ie=UTF8&qid=1417471927&sr=8-1&keywords=kelty+redwing

That's her in my avatar for size comparison.  RPD for scale

Well then I guess we are going to REI...



Interestingly enough, that was her idea to (let's go to REI & try some on)....

No need to bring dog food. Rei keeps weights for this purpose. Go there on a slow day. Get someone to fit her. Fill pack and have her wear it while running around the store.

Pay no attention to the brand or its warranty. Get what she finds comfortable. Rei will take care of you if it breaks in the first year.

Buy the smallest volume you can get her gear into. What seems like extra capability with more pack volume will just end up being extra weight and room to pack unnecessary crap. 50 liters will be too big.

Thats a whole other can of worms........

Fitness and a light pack eliminate the need.

Osprey makes a good womens pack. I would buy whatever model was most comfortable for her. Make sure she walks around the entire store up/down the stairs up/down the rock mound shoe traction thingy.

You could always rent a pack to give it the best test of all. Check here.

My wife started going on multi night trips last year.  We went to rei and tried on quite few. Rei has weighted bags you can through in for weight.

We went with a 65 liter Ospery, and she has been very happy.  It's big enough to carry a bear canister which she has done with it.

Good luck.

Yeah, my wife has tried several packs to include some custom cottage packs (like ULA).  She really prefers and comes back to Osprey packs...but as mentioned, packs are really an individual fit.  Make sure you hit the packs in REI first, get properly fitted, find a comfortable pack and let you wife walk around in REI for an hour or so shopping with the pack on.  

Be careful on REI's recommendations for footwear.  Several times at all the REIs in Atlanta, when we're just looking and the discussion of backpacking comes up, they point out mountaineering boots...heavy, still and really designed for expedition treks or serious mountain hikes with a ton of scree.  Much like the pack, let you wife find the lightest, most comfortable footwear with ankle support if she needs it.  

The best thing about REI is the knowledgeable staff (for the most part) and their return policy.  My son picked out a new (and expensive) Borealis pack when we did a three-day 32 mile section of the AT.  The pack felt great in the store, but didn't work well for him on the trail.  After the trip, my wife took it back and he ended up with an Osprey EXOS.  Pack comfort may be different after a 10-12 mile hike on a rough trail, but chances are if it fits comfortably with weight for an hour or two with weight, it'll probably work for a multiday trip.

ROCK6

Wife humps a Gregory.  Get her fitted at REI +1000.