Edit in JSFiddle

function initialize() {
    var latlng = new google.maps.LatLng(-34.397, 150.644);
    var myOptions = {
        zoom: 8,
        center: latlng,
        mapTypeId: google.maps.MapTypeId.ROADMAP
    };
    var map = new google.maps.Map(document.getElementById("map_canvas"),
        myOptions);
  }

Y.on("domready", function(){
     initialize();   
})

<script type="text/javascript" src="http://maps.google.com/maps/api/js?sensor=false">
</script>
<div id="map_canvas" style="width:100%; height:100%"></div>

  html { height: 100% }
  body { height: 100%; margin: 0px; padding: 0px }
  #map_canvas { height: 100% }