def size_in_bytes(self, offset, timestamp, key, value, headers=None):
        """ Actual size of message to add
        """
        assert not headers, "Headers not supported in v0/v1"
        magic = self._magic
        return self.LOG_OVERHEAD + self.record_size(magic, key, value)
 Actual size of message to add
        
kafka/record/legacy_records.py