It just depends on your wife. Try to arrange for her to handle, operate, and shoot as many different handguns as possible, and have her choose the one(s) she is
a) able to operate (i.e. rack the slide, reach the trigger, reach all the controls, able to hold properly...)
b) able to shoot reliably and accurately (i.e. a big caliber in a small gun might or might not be too much muzzle blast and recoil)
c) WILLING TO CARRY (i.e. not so big or cumbersome that she leaves it at home half the time).
I would much prefer my wife carry our Kahr P-9 or the Kel-Tec .380 but she simply cannot reliably operate the slide -- she has had surgery on her wrists/hands, and she doesn't have the gripping power needed. Smaller automatics often have very stiff springs. She can operate our Glock 17, and I would love for her to carry it, but it is a big gun for her to tote around. So, unlike Roadrunner above, she prefers and currently carries a S&W Airweight Bodyguard revolver.
Your wife may not have the same issues as mine. I have seen a number of gals at shooting courses with the Glock 19, and they can do very well with it. At my wife's CHL requalification shoot last Sunday there was a woman there who had never fired a gun, but after a little coaching from the instructor, she fired a 250 out 250. I don't remember which gun she used, but she should keep that one!
If you can find a range that rents handguns, rent all the ones on the shelf and have her try them. If not, try to find some shooting buddies that have different handguns, and have her try those.
Whatever you choose, make sure you budget for a decent holster and belt. Or pocket holster. Even if she's going to put it in her purse (which I hate, but that's another story), at least have a pocket holster for it to keep it postioned upright, so if/when she needs it, it is READY to go, not upside down, or the handle wedged in the corner or something. And of course the pocket in her pants or purse that the gun goes into should have ONLY the gun in it -- no other doodads to get in the way, or get caught in the trigger guard, etc.
Good luck!
p.s. yes I agree with Amber, have some other (COMPETENT) person teach her, and I highly recommend spending the bucks and going to a name-brand professional, like John Farnam (actually, his wife teaches the basic handgun course, and she is very good. I learned a lot from her). The Farnams will have handgun courses in Victoria at the very end of Feb -- Fri night (27 Feb) to 1 Mar (Sun). The man who organizes the courses in Victoria, Dr Tobin, I think might even be able to arrange for your wife to borrow a handgun and leather gear for the course, so she could go thru the course and then choose a gun. PM me for details.
Having said that, my wife wanted me to train her, and altho there were some tense moments

she soldiered through with me. I was mightily pleased when the CHL Instructor mentioned to me that he was impressed with my wife's gun handling on the range!