B.W.
If her shoulders are over the top harness slot, she needs another car seat ASAP! If the harness goes below her shoulders, she's at serious risk of spinal compression in even a minor collision, even braking too hard. Get her out of that seat right away.
A forward facing car seat is outgrown when one of three things happens, the child hits the weight limit, the ears are level with the seat back, or the shoulders go over the top usable harness slot.
At 5yrs old, she is physically mature enough for a booster seat, but its up to YOU to decide if she is mentally mature enough to keep the belt on at all times (not putting it behind her back), not leaning or reaching, not letting the belt slack, and sitting upright at all times. If she can't do these things, or if she still sleeps alot in the car, you should consider another higher harness weight car seat like the Graco Nautilus. Its $150 at Wal Mart (and they ahve a pink one!) and it will harness to 65lbs and fits most kid in the harness until 6/7yrs old, and then becomes a high back booster, and eventually a no back booster.
If you feel she is mature enough for a booster seat, insist on a high back booster seat. The Graco Turbobooster fits smaller kids very well and is a great seat for the price. Again make sure you get a HIGH BACK booster. A no back booster is not appropriate for a child under age 6/7.