depends on the shape of the board more than the size.
you can have a big guy shortie at 6'"8 or your can have a 6'8" fun shape
or you can have a hpsb at 5'10 and a retro fish or an egg at 5'6", both not at all short boards.
attributes for a shortboard are rail, nose and tail out line, and nose rocker. probably missing some more but those are the few i thought of just now