JimmieT,
Last year one of my 20something nieces came most of the way across the state (a long way here in NC) so I could teach her a session of Defensive Shotgun 101. When we got to the range part of the class, I laid out a selection of various pumpguns on a table for her. She wasn't a neophyte shooter, having used various handguns before, but she was pretty new to shotguns.
We had gone over form and gun mount, and she shot several different makes/models to see what seemed to work for her. What she settled on was a Remington 870 Express Youth in 20 gauge, with a 13" length of pull stock and a 21" vent rib barrel. There was an 870 Express Turkey gun in 12 gauge on the table as well, with a shortened stock and therefore exactly the same dimensions as the Youth gun.
She chose the 20 gauge to run the rest of the class with, not because it recoiled less (she said recoil was about the same as the 12 gauge with reduced recoil loads) but because she found it easier to physically handle- to lift, hold and point. That was because the 20 gauge was lighter in overall weight.
And that's what I meant by saying they're easier to handle by smaller shooters. Recoil is an issue, and the heavier the gun, in general, the less felt recoil is transmitted to the shooter. Using 20 gauge loads with equivalent muzzle velocities to 12 gauge reduced recoil loads (1100- 1200 fps give or take), recoil in the lighter 20 gauge guns should be quite manageable as long as the shooter is in good health and uses good form and a proper gun mount.
As long as she can manage a shotgun, that's what I would suggest for a main defensive weapon. I say that because a long gun is generally easier to manage under stress than a handgun.
But no matter what firearm is ultimately chosen, some good basic training is in order for any new shooter. Please make sure she gets what she needs. I'd suggest pointing her toward
http://www.corneredcat.com/ as a good place to start learning some of the necessary attitudes and information.
hth,
lpl
