What Is A Good Size For Follicles To Do The IUI?

Hi, I am 43 and have had 9 pregnancies-5 live births. I have a new husband who is 42 and has two children. We tried for 8 months to get pregnant and couldn't. So I was put on femura on day 4 and had 2 pills a day for 5 days of femura and had an u/s today showing an 18mm follicle and a 13mm on the other ovary.I am supposed to do the shot tonight and IUI in the am. Are the follicles good size and is that too quick after the shot to do the IUI/