Hi,
Thanks for your query,
Size of follicle gradually increases during 1st half of menstrual cycle. One follicle reaches around 18 to 20 mm ( dominant follicle ), when it becomes mature. After maturation the follicle ruptures and ovulation occurs. Some times follicle does not rupure and increases in size and becomes
follicular cyst. Follicular cyst usually regresses itself with in 3 to 6 months.
Take care and best of luck..
Hope it helps