Newsworthy is a responsive width theme, so how wide the columns (sidebar and main content) are to a visitor is dependent on the width of the browser window and/or the width of the screen it is being viewed on. This is done so that it will adjust to even smaller screens such as on smart phones, and do so seamlessly. Grab your browser window and narrow it way down to see what happens.
To change the theme to a fixed width theme with a content area at 700px would be a pretty big task. It would also mean you would likely have to use the mobile theme for your visitors on smart phones and tablets. You may want to go to appearance > mobile, activate the mobile theme and then visit your site on a smart phone or small tablet to see what it looks like.
If you want to fix the main content at 700px in width, you may want to look at a theme that has a fixed width.
If you want to change this theme to a fixed width theme, you may want to think about using a consultant since it can get quite involved.