Do you want to decrease the content column’s width to do that, keeping the same overall width for your site or do you want to increase the overall width of the site? Right now the maximum width for the content area is set to 1000px (max width because this is a responsive width theme).
How much do you want to increase the sidebar width? The widths are in percentages. Right now the width of the sidebar is 19.68% and the content column is 76.1%.