Try to go to REI when it's not busy, like when they open up. The one in my area is a mad house starting mid morning. Find someone in the packs section and tell them what you are looking for. They'll spend time with you and offer a selection for you to try.

Also, if you don't have one already, I recommend a compression bag for your sleeping bag. It really helps reducing the size, especially if you are carrying a synthetic sleeping bag.

I'll echo what others say but a backpacking type pack really needs to be tried on in a store. If you aren't familiar with the packs, a good store attendant will help you adjust for comfort...there are a bunch of different straps.

"back-packing grandmother" That's bad a$@ !