Since the theme is responsive, it would be good to stick to percentages for width. This example will make the main content area in the Fresh & Clean theme about 640 pixels wide:
This example affects the home page as well as posts.
You should also adjust the content width setting on the Appearance → Custom Design → CSS page to match.
If you have inserted images from outside sources at a fixed width, note that they won't adjust automatically and you may need to remove and re-insert some of the images if you want them to be full width.