Full tilts are narrow. And to convert shoe size to boot size, you add up the digits of the boot size, and that should somewhat correspond to your shoe size. Ex. you wear a 28 boot (2+8=10) so you're around a size 10 shoe. Of course, people wear their shoes big and usually a person with a 28 boot wears an 11 or 12 shoe.
if you ride a 26.5 a 26 should be fine. especially once heat molded.
if im wrong someone more knowledgeable pllease correct me, but i thiink its only the liner size.
and if its too small, selling them shouldnt be a problem.