Pregnyl and clomid

Makensey

So I am on 100mg if clomid and my doctor is letting me take the Pregnyl hah shot. I am going to have an ultrasound on cycle day 15 and see how big my follicles are and then do the Pregnyl shot if they are big enough. Is cycle day 15 too late to have an ultrasound if I took clomid cycle day 3 to 7 and how big have you all heard your follicles need to be? My doc said 1.6 to 1.8 mm that seems a bit small to me or am I wrong?