maybe set a string col itemsize:
               min_itemsize can be an integer or a dict with this columns name
               with an integer size

def maybe_set_size(self, min_itemsize=None):
        """ maybe set a string col itemsize:
               min_itemsize can be an integer or a dict with this columns name
               with an integer size """
        if _ensure_decoded(self.kind) == 'string':

            if isinstance(min_itemsize, dict):
                min_itemsize = min_itemsize.get(self.name)

            if min_itemsize is not None and self.typ.itemsize < min_itemsize:
                self.typ = _tables(
                ).StringCol(itemsize=min_itemsize, pos=self.pos)