Not a specific pack suggestion because there are too many unknown variables.
Your wife is tall enough to have more options than vertically challenged adults BUT in packs that size and larger, choosing one that is more anatomically correct for a woman can be an extremely wise decision. Torso length is torso length, but usually there are important differences in male and female shoulders and hips when it comes to packs. The impact is proportionally less with smaller packs (e.g. day and overnight packs), but it's still there. YMMV; your wife's shape is what actually matters.
The tips/advice here are fair enough:
http://www.rei.com/learn/CampingClick on the links for Backpacks and Woman's Packs
The conditions and location (hence clothing, bag, shelter, food, fuel, water, etc.) matter when it comes to pack size. For that matter, so does the gear she has, from skin out. Smaller/lighter usually costs a lot more - whether it's a sleeping bag or clothing. So applying the "use what you have" principle, make sure the pack is sized appropriately.
As long as she does not succumb to the trap of filling it up because the room is available, too voluminous a pack is generally less of a hassle than too small a pack, despite what the gram weenies say.
Also, it matters if you are proportionately sharing the load with her or if you are the mule or if she intends to solo.
Perhaps the most important thing is to have her actually try (wear) the pack with a least a representative weight in it. (Better is with her actual gear stowed in it). A real outdoors retailer will allow her to bring in her gear when shopping for a pack. A mass market retailer probably will not. Saving 15% - 20% on a pack isn't a bargain an hour into a trip if the loaded pack is uncomfortable or worse
There are (good) arguments for external frame packs and internal frame packs. Try to be open-minded and and not unduly influenced by a [insert type/brand] zealot's claims. This topic is about as bad as "what gun should I get" when it comes to opinions.
My wife and I own and use packs that loosely fall into 3 categories: "conventional" external frame, internal frame with 3 - 5 external pouches/pockets, and "slick" internal frame. We LEAST often use the slick packs, but that is truly more a function of habits and preferences. Packs with no pockets can work fine for an organized and orderly packer.
Go shopping with your wife and have fun. You should easily be able to purchase a quality pack in any style for that amount of money.
Regards,
Tom