Change display size for post image

I’m looking for a way to filter/change the current featured image size from a custom function.

I’ve used this piece of code to force the use of featured images from a looong way back:

function wpforce_featured() {
          global $post;
          $already_has_thumb = has_post_thumbnail($post->ID);
              if (!$already_has_thumb)  {
              $attached_image = get_children( "post_parent=$post->ID&post_type=attachment&post_mime_type=image&numberposts=1" );
                          if ($attached_image) {
                                foreach ($attached_image as $attachment_id => $attachment) {
								set_post_thumbnail($post->ID, $attachment_id);

                                }
                           } else {
                                set_post_thumbnail($post->ID, '414');
                           }
                        }
      }  //end function
add_action('the_post', 'wpforce_featured');
add_action('save_post', 'wpforce_featured');
add_action('draft_to_publish', 'wpforce_featured');
add_action('new_to_publish', 'wpforce_featured');
add_action('pending_to_publish', 'wpforce_featured');
add_action('future_to_publish', 'wpforce_featured');

Works great but throws out the images in full size. I’m looking for a way to change that through filters or a function. The best solution (kind of) I’ve found is this:

function change_image_size( $html ) {
$html = preg_replace( '/(width|height)="\d*"\s/', "width='200' height='50'", $html );
   return $html;
}

add_filter( 'post_thumbnail_html', 'change_image_size', 10 );

This works but it changes the size of the images everywhere – sidebar, footer etc. I just need a way to change the first featured image in every post.

Any help will be appreciated.