OK, I assume that you set the sidebar width to 299px in your theme’s custom CSS? You would do the same thing for the post area.
The main content area is divided into two sections, the post area, which has an ID of primary, and the sidebar, which has an ID of secondary. It looks like you set #secondary to have a width of 299px. The width of the content area is 1020px, so 1020px – 299px = 721px left.
However, the sidebar also has a margin-left and a margin-right of 10px each, so that’s 20 less px: 721px – 20px = 701px left. The post area also has a margin-right of 10px, so now you’re down to 701px – 10px = 691px. So that’s how much space you have left for your post area.
To set the width of the post area, add this rule to your custom CSS:
#primary {
width: 691px;
}
