var viewportwidth;
var viewportheight;
 
 // the more standards compliant browsers (mozilla/netscape/opera/IE7) use window.innerWidth and window.innerHeight
 
if (typeof window.innerWidth != 'undefined') {
	viewportwidth = window.innerWidth+"px",
	viewportheight = window.innerHeight+"px"
}
 
// IE6 in standards compliant mode (i.e. with a valid doctype as the first line in the document)

else if (typeof document.documentElement != 'undefined' && typeof document.documentElement.clientWidth != 'undefined' && document.documentElement.clientWidth != 0) {
	viewportwidth = document.documentElement.clientWidth+"px",
	viewportheight = document.documentElement.clientHeight+"px"
}
 
 // older versions of IE
 
else {
	viewportwidth = document.getElementsByTagName('body')[0].clientWidth+"px",
	viewportheight = document.getElementsByTagName('body')[0].clientHeight+"px"
}

function bigback() {
	document.getElementById("theback").style.width = viewportwidth;
}