If the pack might have to be checked, I wouldn't want a lot of exterior pockets that bulge out and could get hung up on baggage conveyors. One way around that is to have a zippered duffel you can put the pack in for checking.
That's how I travel with my Kifaru Longhunter. Inexpensive REI zippered bag keeps all of the straps and buckles from getting snagged. It worked well for me for 2 weeks in Italy.
Regarding size, something to live out of is going to be tough if it's needed to fit as a carryon. Some smaller planes will do the plane side check for oversized carryon's. If it's to be checked, get the size and fit she likes and a bag to stuff it in.
