Changing "featured image" to look like the large size that's within post.

I’ve searched here and tried to change the image size for the featured image then rebuild thumbnails but it’s not working.

Here’s my site…. http://rockinarugula.com/ and I’d like the feature images to be the same size as they are inside of a post. http://rockinarugula.com/2013/09/17/banana-oatmeal-muffins/

You can see the same image is a featured and inside the post. How do I get it to look more like the photos inside?