20 and 21/22 is really good, IF you were literally about to ov. Did you have ov shortly after those measurements? If you didn't and they got much bigger than that, then the egg was likely "overcooked" for lack of a better term. When ov is getting close you can expect about 1mm growth on the dominant follie per day. Follies much bigger than that... 23-24, maybe 25 is really pushing the upper limits for a nice mature egg. IUI is intra-uterine insemination. You can do it unmedicated (of course for you, continue clomid) where you just monitor for ov using opk. When you get a positive, you go in, dh gives a sample (as if for a SA), the sperm gets "washed" to get rid of all the bad ones (or they can do it w/o washing too), and then the inject the sperm directly into your uterus to give a better chance of getting a bfp. You bypass about half of the obstacles to a bfp b/c sperm doesn't have to pass through cervix, etc.
Otherwise, you can do a medicated cycle, maybe with a higher dose of clomid or a med like Femara or even possibly a low-dose follicle stimulating hormone (FSH) med injecticle like puregon to get a few more follies. Docs usually aim for about 3-4 follies when doing it that way. Again, just to increase your odds of a bfp and then do the IUI. More targets for the spermies. But of course, risk with that is possible multiple pregnancy.
We're not eligible to try IUI at all, mostly b/c my dh's SA is SO poor but also b/c of my endo. The environment in my tubes would be "poisonous" to sperm b/c of the endo and IUI would be unsuccessful.