So are you saying that you want the entire content section to be wider? (That's really the only way to make the comments section wider without making a mess of the layout...) If so, that becomes quite complicated as you will then need to change width and margins on many other elements on whichever templates you are using. Do you want the change to be site-wide or only on the page you linked to?
On that page, the content width / margins are set here:
margin: 0 34% 0 7.6%;
So you could copy the above code to your child theme and make changes. But you'll then likely need to also change this:
A great tool for working with CSS issues like this is Firebug...as it shows you which CSS is affecting a specific element on the page and allows you to test changes in the browser before making them in the actual theme code.