code: public int getSize(DBIDRef id) {
      if(csize == null) {
        csize = DataStoreUtil.makeIntegerStorage(ids, DataStoreFactory.HINT_HOT | DataStoreFactory.HINT_TEMP, 1);
      }
      return csize.intValue(id);
    }
docs: Get the cluster size of the current object.
  
  @param id Object id
  @return Cluster size (initially 1).