I've defined my own folder in the Browser - a list of tables - but - is it possible to order these by size (extents)? If I understand the documentation correctly, then ordering can only be done on columns in the all_objects view .....

You can indeed only use columns from the all_objects view in your folder. You can however create a stored function that returns the table size based on the table owner and name, an use that function in the order by clause of your folder.