Mossberg makes a few nice 18.5" barrel, pump, home defense type shotguns. Personally, especially for a woman, I would not go larger than a 20 gauge. A 20 is very effective at the ranges you would be dealing with in a home invasion, and there is less concern with over penetration. They are also relatively inexpensive and fool proof. My wife brought a 12 gauge double barrel to our marriage which her deceased husband bought for home defense when they used to travel in an RV. I asked her if she'd ever shot it..."No". I then explained to her that she would probably hurt herself trying to shoot the thing, it's heavy as hell, and after the thing kicked the crap out of her on the first shot, she'd never get the second barrel fired. It now sits in a closet.
She now has the Sig P238 on the nightstand. No kids to worry about, so it stays on top of the stand all the time, cocked and locked. On my side of the bed sits a Sig C3 .45 with night sights and Crimsontrace laser grips. Both have extra magazines handy, and both have Hornady Critical Defense rounds in them. We also have an ADT security system with a couple of signs in the yard saying so. Every time a door opens in the house, it beeps twice in two locations, even when not armed. That and six dogs will probably alert me, or make the bad guy choose another home.
Beags...I really think you should check out the Sig P238 for your wife, particularly with her being so small. My wife is 5'3" and about 135lbs, with small hands, and she handles that gun very well. Perfect size for her hands, and the recoil is not bad at all. My .45 is too much for her to handle...she's just not comfortable shooting it.