Follicles and trigger shot?

Chelsey • 2 years of Trying for our 1st Baby, now working with fertility DR. In the midst of the storm,God will be your Sunshine!
Second month on clomid and I went to the doctor yesterday and my ultrasound showed I had 18mm follicle and they gave me my trigger shot, does anyone know how big your follicles are supposed to actually get in order to release the egg/eggs?