there are tens of thousands of different daylilies and many that are 3' or under. You can search the daylily database here on NGA for daylilies of specific heights:
http://garden.org/plants/group...
I'm not sure how to search for all daylilies below 36" though, maybe someone else can help, or you can go to the American Hemerocallis (Daylily) Society database where you can search for all registered daylilies below a specified height:
http://www.daylilies.org/Dayli...
Yes, they can be grown in pots.
Edited to add the AHS database says there are 67,521 daylilies less than 36" tall, although not all will be available in commerce
