code: public static long getRealSize(Resource res, ResourceFilter filter) {
  	if (res.isFile()) {
  	    return res.length();
  	}
  	else if (res.isDirectory()) {
  	    long size = 0;
  	    Resource[] children = filter == null ? res.listResources() : res.listResources(filter);
  	    for (int i = 0; i < children.length; i++) {
  		size += getRealSize(children[i]);
  	    }
  	    return size;
  	}
  
  	return 0;
      }
docs: return the size of the Resource, other than method length of Resource this method return the size
  of all files in a directory
  
  @param collectionDir
  @return