I think for what you're trying to do, you'll need to edit a copule of values within the stylesheet file.  ("style.css")  So, open that file up for editing.

The page is setup as 2 columns, so what you're going to need to do is reduce the width of the left side column, and increase the size of the right side column.  The two widths added together will equal the full width of your website.

The name of the left column is "news" and the name of the right column is "products".

To change the width of "news", search for the style element named "#content .news".

Under that element, you'll see the width in pixels.  (I think standard width is 420px.) For starters, drop that value to "400px".

To change the width of the "products" column, search for the style element named  "#content .products".

Under that element, you'll also see the width.  (I think the default is 400px.)  Now that you've reduced the left column by 20 pixels, increase this width by 20 pixels to make it "420px".

Save the file and see how that changes things.  If you need to make further changes, follow the same formula.