Returns the logical size (before compression) of file `name`.

def logical_size(self, name):
        """Returns the logical size (before compression) of file `name`."""
        digest = self._digest_for_link(name)
        logical_size = self.db.get('{}:logical_size'.format(digest).encode())

        if logical_size:
            return int(logical_size.decode())
        else:
            raise RuntimeError(
                    'Blob doesn\'t have :logical_size in DB: try recovering')