francsutherland on "Internet Explorer 7 - Twenty Thirteen theme - article's not 100% page width"

Hi,
I am using a child theme of twenty thirteen. I have got an IE7 specific css stylesheet which appears to be communicating correctly.
The issue I am having is that the content (both pages and posts) are not filling 100% across the screen as they are with newer versions of IE and other browsers. This is happening in the parent theme too.
Can you tell me what CSS I need to add to my child theme's stylesheet to get it to do 100% width. At the moment it looks like 60% width and centred.
Thanks,
Franc.