This site says they are mostly 8x8' or even 12x12'.
http://www.grundycountyia.com/Quilt_...e/FAQ_page.htm
This one also says 8x8'.
http://develop.hamiltoncountyiowa.co...rn_quilts.html
Barns tend to be very large, and the quilt blocks need to be seen from a distance. I think most of them are community projects, not purchased by individuals.