Page Layout on Rockford theme

I am wanting to increase the body/text area for my blog posts.

Here is an example of a blog post here

It looks as though the body area is around 520px – though I can’t seem to find this when I inspect the element.

However I would like to increase to look like this page but I still need to factor in a side bar. I believe it’s 720px.

How do I makes these changes? And when should I edit?