def chunk_size(self, value):
        """Set the blob's default chunk size.

        :type value: int
        :param value: (Optional) The current blob's chunk size, if it is set.

        :raises: :class:`ValueError` if ``value`` is not ``None`` and is not a
                 multiple of 256 KB.
        """
        if value is not None and value > 0 and value % self._CHUNK_SIZE_MULTIPLE != 0:
            raise ValueError(
                "Chunk size must be a multiple of %d." % (self._CHUNK_SIZE_MULTIPLE,)
            )
        self._chunk_size = value