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