'Looks up the volume named existing and returns its size string.
:param existing: Existing volume dict.
:return: The SC configuredSize string.
:raises ManageExistingInvalidReference:'

def get_unmanaged_volume_size(self, existing):

    vollist = self._get_volume_list(existing.get('source-name'), existing.get('source-id'), False)
    count = len(vollist)
    if (count == 1):
        (sz, rem) = self.size_to_gb(vollist[0]['configuredSize'])
        if (rem > 0):
            raise exception.VolumeBackendAPIException(data=_('Volume    size    must    multiple    of    1    GB.'))
        return sz
    elif (count > 1):
        raise exception.ManageExistingInvalidReference(existing_ref=existing, reason=_('Volume    not    unique.'))
    else:
        raise exception.ManageExistingInvalidReference(existing_ref=existing, reason=_('Volume    not    found.'))
