Here is how I changed the width of my theme. The file is located in whatever theme you are using. My file name was theme.html. Here is the code....
<HEAD></HEAD>
<body background="<!-- [image-path] -->back.gif" leftmargin="0" topmargin="0" marginwidth="0" marginheight="0">
<TABLE WIDTH=800 BORDER=0 CELLPADDING=0 CELLSPACING=0 align="center">
<TR>
<TD>
<IMG SRC="<!-- [image-path] -->alienbonetheme_01.gif" WIDTH=340 HEIGHT=160 ALT=""></TD>
<TD>
<IMG SRC="<!-- [image-path] -->alienbonetheme_02.gif" WIDTH=110 HEIGHT=160 ALT=""></TD>
<TD>
<IMG SRC="<!-- [image-path] -->alienbonetheme_03.gif" WIDTH=350 HEIGHT=160 ALT=""></TD>
</TR>
</table>
<table width="100%" border="0" cellspacing="0" cellpadding="0" align="center" height="100%">
<tr>
<td align="left" valign="top" width="200">
<!-- [left-blocks] -->
</td>
<td align="center" valign="top">
<!-- [center-blocks] -->
<BR />
<!-- [modules] --></td>
<td valign="top" align="center" width="200">
<!-- [right-blocks] -->
</td>
</tr>
<tr align="center" valign="bottom">
<td colspan=3><a href="http://www.predatorial-tendencies.com" target="_blank"><img src="<!-- [image-path] -->alienbonetheme_42.gif" border="0"></a></td>
</tr>
</table>
The width of the first table is 800 while the width of the second table is 100% which will give you the full width of the screen. The first table is 800 becuase that is the size of the header I am using. It should display properly on all screen sizes and then I have the second table at 100 so the content will always take up the entire screen.
I hope this helps you out!
James
www.e-bassangler.com