My personal opinion: Let her take the class first, to make sure she really wants to ride. Then, take her around to all the shops with used motorcycles and have her sit on as many different bikes as possible to see what feels comfortable to her. She'll take over from there, trust me.
More of my person opinion: It is widely written about and talked about that a 250 is the perfect bike to start on. This is probably true IF you plan to really ride the bike and learn to take it to it's limits before moving on to a larger hp motorcycle. However, most people do not have the patience to do this. Yes, you can buy a 250, learn on it for a year and sell it for close to what you paid for it. I don't think most new riders will have the patience to keep it even a year. You have to ask yourself how you feel about this aspect.
It's hard to really know what you want until you've been riding a while and develop your own "style".
I say congratulations for even wanting to ride ... let her choose something and learn on it. Everything else will fall into place as she goes along.
Good luck ... be sure and keep us updated on her progress!