Usually, the wide screen posts are the result of a picture or contiguous text string, like a URL. This is the main reason I use the UBBCode to shorten URL's to plain words and link to pix instead of displaying them in posts. That and it cuts down on the bandwidth at my web site since the only time pages or pix are brought up are when a VMF'er clicks on the link.
The widest you want to go is about 7 inches (or about 500 pixils) at 72 DPI. 6 inches is even better. That way it will always fit on a monitor, even if it's set to a low resolution. The 72 DPI part is important as that is web standard for images and the monitor doesn't see inches, only pixils.