If something is petite, it's tiny. Your mom might be so petite that she needs to sit on a telephone book when she drives the car.
The adjective petite is used to describe a small woman. When you call someone petite, it's usually meant as a compliment — implying that she is dainty and adorable. Many clothing stores offer petite sizes for women who are short. The word petite is the feminine form of little in French, and sometime in the 1700s it became used frequently in English literature to describe those who are small in stature.