You've already had a bunch of responses.
Ask yourself:
What is her level of experience, if any?
Does she know the differences between a revolver and an autoloader, at least to the point that she can make an informed decision?
Has she fired any guns, let alone a variety of calibers?
If not, I'd suggest that you get her to a range that has rentals available (or perhaps you have one of each of the likely guns she would choose).
Size is certainly not a limiting factor in shooting. My wife is about the same size as your wife, and she will shoot anything: 10ga slugs out of a light single-shot shotgun, a .375 H&H Mag, etc... Your wife doesn't want anything too big and powerful, but has she shot enough to equate that perception to something meaningful?
Anyway, I won't provide specific weapon recommendations, as all of these variables are a factor. For me, reliability is job one, and simplicity is a close second...
My wife carries a Glock 27 (40S&W). It works as a purse gun, has enough power, and holds 9+1. And perhaps of greatest importance: The gun fits her, she is comfortable with it, and shoots it well.