In September 2013, graphic designers were given a terrific gift from Facebook. News feed thumbnails for links were enlarged to 400 pixels by 209 pixels. Authors can now use a comfortably large space to visually promote links to their articles.
Make sure your WordPress template is coded to use your WordPress post’s featured image as the source for Facebook’s OpenGraph image meta tag <og:image>. Then, when it comes time to create a thumbnail asset for your article, you or your graphic designer can use the new dimensions. When the link to your post is publicized to Facebook, it will display the larger image as the article’s graphic. The Jetpack plugin by WordPress can handle both adding the OpenGraph tag and publicizing your post to Facebook. In fact, if you arrived at this post from Facebook, that’s how you did it.
I use a graphic template that is 398×208, just slightly smaller than the Facebook specification. Facebook is constantly tweaking their page layout, so the actual image window as of this writing is 377 pixels wide.