The most important thing is that the gun feels comfortable in your hand. You should test out multiple weapons to see which one feels the most natural. You will probably want to stay away from heavy guns and focus on lightweight models, although keep in mind that in general, the longer the barrel, the more accuracy you will have.
It depends on her size (especially her hand), but since most women are "light" I would say a 9 mm. With any gun though, they should fire it first to see how comfortable they are with it.
