The standard (my decision) bandana handkerchief is 21" x 21".
See Drysdales Western Wear-
http://www.drysdales.com/Mens/Neckties-Wild-Rags/detail-355-Basic-Red-Bandana.html
They sell yer Basic Red for US$1.50. And it is 21" x 21" (furriners do their own metric figuring).