seems the route with the most success will have either to do with

1. givin up the idea of surprising her with a bike and having her get involved with the process or
2. look up how to fit a mountain bike, get her measurements, do a ton of reading and take a best guess.

one place you can learn about mountain bikes is mtbreview.
http://www.mtbr.com/
another place you might learn about fit is wombats
http://www.wombats.org/tips.html

she'll love you for the thoughtfulness it takes to make sure it fits.
IF it doesn't fit...she'll feel all guilty about being honest, you'll feel like a heel for being so impatient. been there.