Q: What is the size of the leading follicle before HCG injection?

A: The largest follicle ("leading follicle") is usually the follicle from where ovulation will occur.
Follicles usually grow 1-2 mm per day, so a follicle that's about 188 mm or so will probably be the one where ovulation will happen within one to three days.
There is no uniform opinion as to the required size of the leading follicle. Many doctors think the time for an hCG injection has come when the follicle has reached a size of 18 mm by transvaginal ultrasound.