Gonal-f, HCG trigger with IUI?


Hello All,
I am on Gonal-f,HCG Trigger with IUI.
My 1st IUI was fail and going to start 2nd round.
I just want to know that What is a good mature follicle size for trigger when you are on Gonal-f?
My 1st IUI, i triggered on 16.5mm, When i read/search on forum they said “Mature follicle size between 18-20mm for trigger according to her Clinic”.
Whenever i asked my Clinic they said we consider good follicle size is 16mm for HCG trigger. I really confused and please clear my doubt.Thank you!
your help would be greatly appreciated!