I know I can set up post, pages, and custom post types to use featured images in wordpress by enabling theme support:
Or enabling theme support for a specific set of post types:
add_theme_support( 'post-thumbnails', array('post','page'));
I also know that I can add an "image size" like this:
add_image_size( 'media_library', 333, 230, true );
But how do you control which image size of the "feature image" actually displays on the edit screen of Wordpress? For example in TwentyEleven I have uploaded a square photo and it shows up as a rectangle in the featured image box on the edit screen:
How do you pick which image size shows up there?