Return the ``<c:bubbleSize>`` element for this series as an oxml
        element. This element contains the bubble size values for this
        series.

def bubbleSize(self):
        """
        Return the ``<c:bubbleSize>`` element for this series as an oxml
        element. This element contains the bubble size values for this
        series.
        """
        xml = self._bubbleSize_tmpl.format(**{
            'nsdecls':    ' %s' % nsdecls('c'),
            'numRef_xml': self.numRef_xml(
                self._series.bubble_sizes_ref, self._series.number_format,
                self._series.bubble_sizes
            ),
        })
        return parse_xml(xml)