Take her to the range & let her try several. My wife has very small hands & what I thought was good for her, wasn't even close. Also, shot placement is more important than caliber....she needs to be comfortable with what she chooses.
Excellent advice. My wife was working in another town and a guy started stalking her. I went to the gun store and bought her a KelTek P-11 9mm. She hated it. She fired it once and the recoil was too much for her (it weighs about 14 ounces empty).
She eventually selected my favorite pistol as her choice for home defense. It's a Colt Diamondback 38 special with custom grips. It fits her hand well, it has manageable recoil, and she shoots it very well. It's simple to operate and doesn't require a lot of practice.
I could have saved some money (to buy a Glock) if I had let her choose her own weapon.