In your code you don't show any attempt to set the width of the body. However, you can either set it from the HTML markup as,
<body style="width: 50in;">
<!--
</body>
.. or you can use the CSS stylesheet to add the document stylings, as
body {
width: 500in;
}
..but you will need to link this stylesheet to the Master page for styling the pages with this style.
There are many other options that you can use, to set the Width of the body element, some might be ASP.NET client-side Css codes etc, but the above ones are an easy way.