Ovarian follicle development proceeds independently of the menstrual cycle, and follicles therefore range from .003 millimeters to 20 millimeters on the 10th day of a cycle, according to the Global Library of Women's Medicine, or GLOWM. Mature follicles that are ready for ovulation are close to 20 millimeters in size.
The development of ovarian follicles from precursor to mature oocyte takes approximately 300 days, writes GLOWM. Therefore, any follicle that has developed sufficiently to be ready for ovulation in a given cycle is near the mature size of between 16 and 20 millimeters in diameter. There are usually several such follicles at any given time, but only one is selected as the primary follicle and is involved in ovulation during most menstrual cycles.