How To Test Follicle Size?

Hi. I took clomid(first time one tablet twice a day for 5 days sarting the 3rd day of my cycle) and was given a trigger shot on day 13. My doctor said that i responded very well to clomid and i had many growing follices but two weregood size for the trigger shot (i think one was 22 and the other 25) . My question is how do u know what is the appropriate size?