def add_size_info (self):
        """Set size of URL content (if any)..
        Should be overridden in subclasses."""
        maxbytes = self.aggregate.config["maxfilesizedownload"]
        if self.size > maxbytes:
            self.add_warning(
              _("Content size %(size)s is larger than %(maxbytes)s.") %
                  dict(size=strformat.strsize(self.size),
                       maxbytes=strformat.strsize(maxbytes)),
                tag=WARN_URL_CONTENT_SIZE_TOO_LARGE)