docs: def measureSize(self, diff, chunkSize):
          """ Spend some time to get an accurate size. """
          (toUUID, fromUUID) = self.toArg.diff(diff)
          isInteractive = sys.stderr.isatty()
          return self.toObj.diff(self._client.measureSize(
              toUUID,
              fromUUID,
              diff.size,
              chunkSize,
              isInteractive,
          ))