KDE 4... It's the biggest WM/DE available. Though Gnome is constantly fighting for that title
Debian's KDE4-minimal is about 15mb compressed, plus qt4, plus other deps. Might be more, I didn't follow all the KDE-specific deps.
edit: That's unfortunately not a normal window, but a Plasma widget. Though I believe KDE4 can be configured like that for windows as well.