I got distracted by the title of this thread, and answered it incorrectly. The correct answer is "the largest gun that she can shoot comfortably and still conceal" - same as for a man.
After all, why carry? In case you need it. If she's going to carry responsibly and effectively, she will need to train and practice with her gun. If a gun is too small (in any dimension) or too light weight, it is less comfortable to shoot. If it is too uncomfortable, it won't get shot much or well. A larger grip reduces felt recoil, spreading it over a larger area. It also is a real help and comfort to those with a touch of arthritis - common in 50-something females in my family.
My wife started shooting handguns a year ago, but missed much of that due to shoulder surgery. When she first started, she shot a .38 Airweight snubbie pretty well, a .22lr snubbie pretty well, a 5" 686+SSR very well, and a full-size Colt stainless 1911 extremely well. My 22/45 was not a favorite for her - didn't balance well for her. She shot a .22LR converted G17 well, though.
We (well, mostly me) decided she should have her own, and she chose a nice, slim LC9 after holding someone else's at the LGS. Took a while to get one, and when we did she was excited - until she shot it. Three rounds and she was done. It punished her hands. Tried a grip sleeve, still not good. Rented a bunch of 9mms, and while she shot the G19 marginally better than the M&P9c, the M&P was much more comfortable for her. She's very happy with her 9c, and the new owner of the LC9 is happy, too.
The 9c is about as big as she can handle well. With Gold Dots, it is as much recoil as she wants, where the .40 is too much. The weight is just right, where the all-steel guns are too heavy for her. She prefers the balance of the 9c over the 9 full size - she shot both, and the choice was clear. She has learned to hold it close and push the grip to cycle the slide, and does that well now. She enjoys shooting her gun, and she shoots it well.
I have half a dozen 9mms that I shoot better than the 9c - and that doesn't matter. What matters is what she shoots well and is comfortable with - so she shoots often. Every time she shoots, she improves. She has gotten to the point that she corrects her own form, and I hear her mumble "It's all about the trigger" every once in a while - and she improves. Next it will be "slow is smooth, smooth is fast"....