Windowssize in CSS, automaticly

I'm using in the Learning Object Titke a Stylesheet for the Windowssize usib=ng Default as Display mode:
body {
text-align: center;
}

#x_mainHolder {
width: 900px;
height: 700px;
position: absolute;
z-index: -3;
}

But, this fixed size is not automaticly shown on the screen. You have to dor this manually, or by using F11. When the user always has to do this, it's not good for the usability of XOT.
Is it possible that XOT shows the window automaticly using CSS-sizes?
Ronald