Opening a window fullscreen is not as easy as you might expect. If the only browser out there were Internet Explorer, then you would have no problem, but unfortunately (or fortunately) this is not the case.
Take a look at this previous discussion, detailing how you might go about detecting browser resolution and then opening a suitably sized window.
How to detect resolution
Hope that helps!
Paul
NB. A cautionary note... not every visitor will take too kindly to you taking control of their browser window. Good practice dictates you should alert people to this fact before the event! :o)