IUI question

So I’m going to be starting IUI soon . I’m waiting for

My next period to arrive then I’ll start the process of it . My doctor wants me to take clomid cycle day 3 - 7 , then on cycle day 10 I get an ultrasound to see the size of my eggs . So my question is what if my eggs aren’t at the right size yet ? How long will it take for them to be the correct size . When they are at the right size I have to inject the hcg shot.