validate this column: return the compared against itemsize

def validate_col(self, itemsize=None):
        """ validate this column: return the compared against itemsize """

        # validate this column for string truncation (or reset to the max size)
        if _ensure_decoded(self.kind) == 'string':
            c = self.col
            if c is not None:
                if itemsize is None:
                    itemsize = self.itemsize
                if c.itemsize < itemsize:
                    raise ValueError(
                        "Trying to store a string with len [{itemsize}] in "
                        "[{cname}] column but\nthis column has a limit of "
                        "[{c_itemsize}]!\nConsider using min_itemsize to "
                        "preset the sizes on these columns".format(
                            itemsize=itemsize, cname=self.cname,
                            c_itemsize=c.itemsize))
                return c.itemsize

        return None