hai provato cosi?

function resizeDiv(){
height = 0;
width = 0;
if (document.all){
height = document.body.clientHeight;
width = document.body.clientWidth;
document.all.livello.style.height = (height);
document.all.livello.style.width = (width);
}
else if(document.getElementById && !document.all){
height = window.innerHeight;
whidth = window.innerWidth;
document.getElementById("livello").style.height = (height);
document.getElementById("livello").style.width = (width);
}

if (window.screen.height > 600 && window.screen.width > 800){
resizeDiv(100, 200);
}
}