i would have her get some hands on experience first and see what she likes. smaller isnt necessarily better. the smaller the gun usually the more felt recoil. i know alot of people that cant rack the slide on a semiauto. i would let her try a few revolvers and a few semiautos and let her decide. she is the one that will be carrying it so she needs to pick it out. good luck with your decision.
+1If anyone hasn't posted it, I will:
The best part about "gun shopping" is just that. Let her make the choice, hopefully you can find a range/shop that rents weapons and let her try them out for herself.
It certainly helps if she has confidence in the weapon and has some confidence in her ability to manipulate it.
Ain't gun shopping fun???