code: public long getNodeDataSize(String nodeIdentifier) throws RepositoryException
     {
        final WorkspaceStorageConnection con = dataContainer.openConnection();
        try
        {
           return con.getNodeDataSize(nodeIdentifier);
        }
        finally
        {
           con.close();
        }
     }
docs: Returns node data size. See for details WorkspaceQuotaManagerImpl#getNodeDataSize(String).
  
  @param nodeIdentifier
  node identifier which size need to calculate
  @throws RepositoryException
  if any exception is occurred