Thanks for both responses. I read the post that you included and it was a little help, as there were a few references to a 48cm being about right.

FYI, her cycling inseam was measured with socks and feet 8" apart with the "Notebook test" applying some pressure with a 3 ring binder in the crotch and then measuring from the top of the binder to the floor. As far as flexibility and fitness, she is an ex gymnast and very fit/athletic (she is going to be a good cyclist, I can tell from spin classes she has been doing with my fiance and me).

As I have seen a few recommendations for a 48cm, my questions are this:

What top tube length do I need to focus on?

Would a square geometry frame (48 seat tube and 48 top tube) be the right way to go, or should I try and find something a bit shorter, like a 47cm or even 46cm?

Do I need to be looking for 650c wheels for a frame this small to have good handling?

Due to her budget, I am looking at $400 to maybe $500 range. I would like to go used to get her a little better group (105 if I am lucky), etc, since Sora at $400 - $500 is about impossible these days.

Again, this is a huge help for me, so thank you all for your input!