Two things:
1.) Stand her next to the bike. If the seat is in a lower position, and it's just below her waist, you're in the ball park. At, or above her waist, too big.
2.) I really get tired of hearing from parents who want multiple years of service from a cheap bike. Forget about "her outgrowing it in a year", and put her on what fits -- this is your CHILD, she's not worth it? My 14-y-o daughter is two years into her fifth bike; she's stopped growing, but I kept her in the right size for her as she grew, and she had some sweet rides! (I had access to upgrade parts)