Based on her weight, a board in the range of a 133-138cm length would be what I'd recommend. She really wouldn't need to go any bigger than that. A longer board is going to be somewhat more difficult to control the way she'd like when she wants to get more aggressive on the water. It will work fine at first, but feel a bit too big as her abilities progress. You can get a really nice package deal on a Trip 133 (or 138) with Alpha bindings that would be an excellent setup for her, but the recommendations above are all good, too. It depends on what style of ride you think she'll like best. Will she want a board that has pretty solid tracking ability, or a little looser feel?
See Dick Ride Wakeboards www.seedickride.com