title: Size of featured image in admin panel?
body: <p>Been experimenting with some custom meta boxes, but nothing I've read or can find is helping me to achieve what I'm actually after.</p>
  
  <p>I can render a new meta box, but I can't find a way to actually change the output of the post thumbnail itself in the admin panel.</p>
  
  <pre><code>function new_post_thumbnail_meta_box() {
      global $post; 
  
      echo 'Content above the image.';
  
      $thumbnail_id = get_post_meta( $post-&gt;ID, '_thumbnail_id', true ); 
      echo _wp_post_thumbnail_html( $thumbnail_id ); 
  
      echo 'Content below the image.';
  }
  function render_new_post_thumbnail_meta_box() {
      global $post_type;
  
      // remove the old meta box
      remove_meta_box( 'postimagediv','post','side' );
  
      // adding the new meta box.
      add_meta_box('postimagediv', __('Featured Image'), 'new_post_thumbnail_meta_box', $post_type, 'side', 'low');
  }
  add_action('do_meta_boxes', 'render_new_post_thumbnail_meta_box');
  </code></pre>
  
  <p>How to have a larger or 'actual size' version of the featured image displayed in the administration panel?</p>
  
  <p>Copied the includes function and modified the output, this throws up a server error.</p>
  
  <pre><code>function _wp_post_thumbnail_html( $thumbnail_id = null, $post = null ) {
              global $content_width, $_wp_additional_image_sizes;
  
             $post = get_post( $post );
  
              $upload_iframe_src = esc_url( get_upload_iframe_src('image', $post-&gt;ID ) );
              $set_thumbnail_link = '&lt;p class="hide-if-no-js"&gt;&lt;a title="' . esc_attr__( 'Set featured image' ) . '" href="%s" id="set-post-thumbnail" class="thickbox"&gt;%s&lt;/a&gt;&lt;/p&gt;';
              $content = sprintf( $set_thumbnail_link, $upload_iframe_src, esc_html__( 'Set featured image' ) );
  
              if ( $thumbnail_id &amp;&amp; get_post( $thumbnail_id ) ) {
                      $old_content_width = $content_width;
                      $content_width = 600;
                      if ( !isset( $_wp_additional_image_sizes['post-thumbnail'] ) )
                              $thumbnail_html = wp_get_attachment_image( $thumbnail_id, array( $content_width, $content_width ) );
                      else
                              $thumbnail_html = wp_get_attachment_image( $thumbnail_id, 'post-thumbnail' );
                      if ( !empty( $thumbnail_html ) ) {
                              $ajax_nonce = wp_create_nonce( 'set_post_thumbnail-' . $post-&gt;ID );
                              $content = sprintf( $set_thumbnail_link, $upload_iframe_src, $thumbnail_html );
                              $content .= '&lt;p class="hide-if-no-js"&gt;&lt;a href="#" id="remove-post-thumbnail" onclick="WPRemoveThumbnail(\'' . $ajax_nonce . '\');return false;"&gt;' . esc_html__( 'Remove featured image' ) . '&lt;/a&gt;&lt;/p&gt;';
                      }
                      $content_width = $old_content_width;
              }
  
              return apply_filters( 'admin_post_thumbnail_html', $content, $post-&gt;ID );
      }
  </code></pre>
  
  <p>Or should I be using add_image_size( 'post_thumbnail', 600, 400, true ); ? since that's what it looks for?</p>