Re: Responsive Thumbnails in WordPress

[noahgelman]

You can name the size of the featured image to be displayed in the functions.php where you activated the featured image option.

Another way which gives you more flexibility is to use the Multiple Post Thumbnails plugin. Not only does it let you set the size of your featured thumbnail, but it allows you to create multiple versions of the featured thumbnail. You can have a small one for sidebar posts, a medium for content posts, and a large one for individual posts, and stuff like that.