def blob_size(self, digest):
r = self._request('<STR_LIT>', '<STR_LIT>' + digest)<EOL>return long(r.headers['<STR_LIT>'])<EOL>
Return the size of a blob in the registry given the hash of its content.

:param digest: Hash of the blob's content (prefixed by ``sha256:``).
:type digest: str

:rtype: long
:returns: Whether the blob exists.