Allow her to choose! And I STRONGLY suggest you give her the opportunity to try an IGH bike also. I helped my wife choose her first multi speed bike, it was a 24 speed GT Slipstream, 700c wheels, step thru frame. She rode it, but had trouble figuring out which gears to use when and which shifter did what. Fast forward about 6 years, we "won" a 1972 Raleigh Colt in her size (17") with a step thru frame. It has become her favorite bicycle because it is a hop on and ride and all you do is shift up or down, she can wear a skirt riding it....YMMV So now we will be exploring getting an upgrade to a 5-8 speed IGH bike.

Aaron