Update size.

def setSize(self, size, sizeIsEstimated):
        """ Update size. """
        self._size = size
        self._sizeIsEstimated = sizeIsEstimated

        if self.fromVol is not None and size is not None and not sizeIsEstimated:
            Diff.theKnownSizes[self.toUUID][self.fromUUID] = size