code: def _set_size_code(self):
          
          if not self._op.startswith(self.SIZE):
              self._size_code = None
              return
  
          if len(self._op) == len(self.SIZE):
              self._size_code = self.SZ_EQ
          else:
              suffix = self._op[len(self.SIZE):]
              self._size_code = self.SZ_MAPPING.get(suffix, None)
              if self._size_code is None:
                  raise ValueError('invalid "{}" suffix "{}"'.format(self.SIZE, suffix))
docs: Set the code for a size operation.