Look for the first line of the following code inside the HTML editor and play with the values highlighted in black:
<b:template-skin>
<b:variable default='930px' name='content.width' type='length' value='1000px'/>
<b:variable default='0' name='main.column.left.width' type='length' value='0px'/>
<b:variable default='360px' name='main.column.right.width' type='length' value='402px'/>
That's it!
Not that this would be a recommended thing for most sites, of course. However, if you want to do something of an artistic nature, which only folk with high screen sizes could see in full - this would be how, I guess... ;-)