Te paso tu función modificada:
Código Javascript
:
Ver originalwindow.onload=function(){
var x = 20;
var box=document.createElement("div");
box.style.top="50px";
box.style.left="50px";
box.style.width = x+"px";
box.style.height = x+"px";
box.style.backgroundColor = "blue";
box.style.position = "absolute";
box.addEventListener("click", function(evt){
this.style.width = '200px';
this.style.height = '200px';
}, false);
document.body.appendChild(box);
}