If other people in the family are petite, than no 'instrument' is going to work. What is done in Pediatrics when there is a question of proper growth is to plot the height and weight on a graph. That combined with genetic issues, like normal size in the family will help the doctor assess what needs to be done. He/she may need to do blood tests to see if there is a growth hormone deficiency
I am unaware of any instrument and would not encourage it if there was. She needs to be seen by her doctor.
Carol Rogala D.O.