by width of main content column which area do you exactly mean?
Check out this example. It shows the web inspector in the Chrome browser. To find it, I right-clicked on the main content area on the http://letstrycreating.com/tutorials/ page then I looked in the HTML panel on the left until I found the container for the content. When you click on the container HTML element on the left, you will see all of the CSS that applies to the element you clicked on in the right panel. So, in this example, you can see how the theme has used the “.content-area” selector and set the width to 42.7451%