Using the Virtual Earth Map Control API, it is very simple to create a mashup application that is full screen and takes up the entire browser window. 

Here is the source code for creating a v6 Virtual Earth full-screen mashup:

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html>
<head>
<title>Virtual Earth v6 API - Full Screen Sample</title>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<script type="text/javascript" src="http://dev.virtualearth.net/mapcontrol/mapcontrol.ashx?v=6"></script>
<script type="text/javascript">
var map = null;

function CreateMap()
{
     map = new VEMap('myMap');
     map.LoadMap();
}
</script>
</head>
<body style="width:100%;height:100%; margin:0 0 0 0;" onload="CreateMap();">
<div id='myMap' style="position:absolute;top:0px; left:0px; width:100%; height:100%;"></div>
</body>
</html>

Here is a link to the complete full-screen example:

Virtual Earth v6 Full Screen Sample

Many people have asked about this since there is a slight difference for v6 full screen mashups than in the previous v5 version.  In the v5 version, you did not have to set the style on the body to make it full screen -- you would only have to set it on the map DIV. This is because the v5 API would set the width and height of the body by default to 100%.  However, in the v6 API we have decided to remove the width and height settings on the body since the Virtual Earth map control does not own the body element of the page and not all mashup applications will want the height and width of the body set to 100%.  Because of this change, if you only set the map DIV width and height to a percentage, this percentage may not be of the full screen because it depends on the parent container width and height -- which in this case is the body.  Setting both the body width and height and the map DIV width and height to 100% will make the mashup full screen.