To increase one's own height, a mantra to Ganesh, the remover of obstacles, would be appropriate: "Om Gum Ganapatayei Namah," for example. For a mother who wishes to increase the height of her child, a mantra to Shakti, or Lakshmi, would be more suitable.
In the Hindu tradition, it is believed that mantras have the power to support and transform the world and its people. Sanskrit mantras are often associated with specific Hindu deities, of which there are many. While there is no specific mantra associated with increasing a person's height, there are many mantras associated with Ganesh (the remover of obstacles), Shakti (the mother, the powerful feminine), and Lakshmi (goddess of prosperity, wealth and good fortune). Any mantra associated with one of the above deities would be appropriate, provided that the desire for increased height is based on positive spiritual grounds.
The practice of repeating a mantra is called japa, and it is through repetition that this power is derived. In strict terms, the selection of a mantra is a sacred thing: a mantra is often handed down within a family across generations, or may be assigned to a student or spiritual aspirant by a guru (teacher). However, the power of mantra and japa is not believed to be affected by those factors.