Make posts show wider and sidebar more to the right?

OK, so I’ve read enough in here to know for some reason the overall WIDTH of the content area cannot be easily changed. That is the only thing wrong with this theme in my opinion so I installed a very popular ‘Custom CSS’ plugin…
My posts are going to be long and full of info so I’d LOVE to have more room rather than have everything stacked.

Can anyone give me the code to add to the plugin to make the site content area (posts, page content, etc.) become wider?