Well there are actually practical limits on a variety of entities, like size of identifier names, numbers of methods, limits on parameter lists, etc; So there are upper limits on how big a class file can be.
Details here:
http://java.sun.com/docs/books/vmspec/2nd-edition/html/ClassFile.doc.html#88659 (Thanks to zakaria haque for the link)
[ March 12, 2002: Message edited by: Rob Ross ]