'Get the size of an unmanaged volume on the Datera backend
The existing_ref must be either the current name or Datera UUID of
an app_instance on the Datera backend in a colon separated list with
the storage instance name and volume name.  This means only
single storage instances and single volumes are supported for
managing by cinder.
Eg.
existing_ref == app_inst_name:storage_inst_name:vol_name
:param volume:       Cinder volume to manage
:param existing_ref: Driver-specific information used to identify a
volume on the Datera backend'

@datc._api_lookup
def manage_existing_get_size(self, volume, existing_ref):

    pass
