I had the same trouble, till I found this:
http://www.jirka.org/gdm-documentation/x1454.html,
it has explained everything there, so used.
I have a doubt, I use mandriva 2006, and when I put a theme with the userlist visible, this show in the top left corner very narrow and tyni, and I change the x and y and heigh and weigh, in the xml file, but nothing happend, it still the same, any idea.....
Greetings from Ronald