Well, it's about how much leverage there is on the steerer... so, given it's for your wife, and any general dimension recommendations are gonna be made with a big guy in mind as the worst-case scenario, I'm sure you can stretch it, maybe 50% even, given how much less grunt is likely involved in this case.

Throw it together how you'd like to build it, and get the wife to put some torque through the bars and observe.