Consider a waterproof pack with zipper pulls that can be opened with mittens if you plan to be out in the rain or snow. Check the bag for durability by looking for solid stitching around the zippers, reinforced bottom panels and reinforced straps. Try various backpacks on for fit at a reputable outdoor store with a salesperson willing to spend a bit of time with you. A salesperson will be helpful in suggesting packs and adjusting them for maximum comfort. Consider renting a pack and trying it out before purchasing it.
Many outdoor stores offer a range of rentals.
Most backpacks are adjustable, but try to buy a pack closest to your size. Avoid backpacks that try to do too much - the more complicated the pack, the more things there are to snag, catch or malfunction. And remember, you pay a great deal more for extra features. Packs fit best when full, and if you have extra room, you're bound to end up carrying the extra loads of your companions. Don't purchase a pack before trying it on first.