Personalizar Post especifico do Loop??????

Use a mesma lógica que eu sugeri neste post:
http://br.forums.wordpress.org/topic/publicidade-no-meio-de-dois-posts-do-categoryphp?replies=2

Para limitar o número de caracteres, coloque esse código no functions.php:

function the_content_limit($max_char, $more_link_text = '(more...)', $stripteaser = 0, $more_file = '') {
    $content = get_the_content($more_link_text, $stripteaser, $more_file);
    $content = apply_filters('the_content', $content);
    $content = str_replace(']]>', ']]>', $content);
    $content = strip_tags($content);
   if (strlen($_GET['p']) > 0) {
      echo "<p>";
      echo $content;
      echo " <a href='";
      the_permalink();
      echo "'>"."Read More →</a>";
      echo "</p>";
   }
   else >if ((strlen($content)>$max_char) && ($espacio = strpos($content, " ", $max_char ))) {
        $content = substr($content, 0, $espacio);
        $content = $content;
        echo "<p>";
        echo $content;
        echo "...";
        echo " <a href='";
        the_permalink();
        echo "'>".$more_link_text."</a>";
        echo "</p>";
   }
   else {
      echo "<p>";
      echo $content;
      echo " <a href='";
      the_permalink();
      echo "'>"."Read More →</a>";
      echo "</p>";
   }
}

Depois chame assim:
<?php the_content_limit(20, "[Read more]"); ?>