so gehts zumindest beim IE (NN keine Ahnung...)
<SCRIPT LANGUAGE="JavaScript">
function handlerMM(e){
Xmm = (isNav) ? e.pageX : event.clientX;
Ymm = (isNav) ? e.pageY : event.clientY;
document.getElementById("myDiv").style.width = Xmm-100;
document.getElementById("myDiv").style.height = Ymm-100;
}
document.onmousemove = handlerMM;
</script>
<br><br>
<style type="text/css">
#myDiv {
border-width:1px;
border-style:solid;
border-color:red;
}
</style>
<div id="myDiv" style="position:absolute; left:100px; top:100px;"> </div>