'Returns the size of volume to be managed by manage_existing.
When calculating the size, round up to the next GB.
:param volume: cinder volume to manage
:param existing_vol_ref: existing volume to take under management
:returns: the size of the volume or raise error
:raises VolumeBackendAPIException:'

@cutils.trace
def manage_existing_get_size(self, volume, existing_vol_ref):

    return self._manage_existing_get_size(existing_vol_ref)
