window.html

<script src="http://ajax.googleapis.com/ajax/libs/jquery/2.0.0/jquery.min.js"></script>

<style>
#bg-left-extension {
  width: 1000px;
  background: blue;
}
</style>

<div id="bg-left-extension" height="100"></div>

<script>
  function getWindowSize() {
		var w = 0, h = 0;

		if(typeof(window.innerWidth ) == 'number' ){
			w = window.innerWidth;
			h = window.innerHeight;
		} else if(document.documentElement && (document.documentElement.clientWidth || document.documentElement.clientHeight)){
			w = document.documentElement.clientWidth;
			h = document.documentElement.clientHeight;
		} else if(document.body && (document.body.clientWidth || document.body.clientHeight)){
			w = document.body.clientWidth;
			h = document.body.clientHeight;
		}

		return {width: w, height: h};
	}

  $(document).ready(function() {
    size = getWindowSize();
    $("#bg-left-extension").css('height', size.height);
  });

  $(window).resize(function() {
    size = getWindowSize();
    $("#bg-left-extension").css('height', size.height);
  });
</script>