<div class="big_content_item">CONTENT</div>

<div class="wide_content_item">CONTENT</div>

<div class="content_item">CONTENT</div>

[big_content] или [wide_content] или [content]

add_filter('the_content', 'filter_content');
function filter_content( $content ){
    return $content . ' Конец!';
}