'Return size of an existing volume.'

@cinder_utils.trace
def manage_existing_get_size(self, volume, external_ref):

    target_vol_name = self._get_existing_volume_ref_name(external_ref)
    cli_vol = self.rdxApi.find_volume_by_name(target_vol_name)
    return int((cli_vol['size'] / units.Gi))
