My day 12 follicles were 15 mm, 18 mm and 22 mm. My RE said he likes to see sizes more than 20 mm for iui.
When I did IVF, my doc wanted to see my follicles around 18-20mm. Good luck!
they are suppose to get bigger then on the day of ovulation the follicle that has ripened the most or is the most mature will be the one to leave the ovary to be fertilized for a baby and the others don't