Tell her you're taking her out for lunch on her birthday...and then pull up in front of the Bicycle Shop...give her a "gift certificate" and let the Shop fit her on the bike of her choice....
I just gave a bike as a gift and ALMOST got it right...a little small, and it has caused some consternation trying to make it right...I wish I'd done what I just told you to do!!!
It's such a personal thing, a bike...