It seems to be doable, look here for example: http://stackoverflow.com/questions/445893/create-window-larger-than-desktop-display-resolution. (Search for the answer that contains SWP_NOSENDCHANGING.)
Anyway, Windows don't allow windows larger than screen (unless you bypass it using the above-mentioned solution - which I personaly haven't tried!) and that's probably for a good reason, so I personally would follow that restriction and prevent the user to choose a larger window than possible. Regardless how complicated the code would be ;)
A window larger than screen may actually behave badly, because the OS doesn't expect to have such windows. You won't be able to resize it by dragging the borders at the very least.