Change width of single posts

Hi, I have a little problem trying to setting the width for just the single posts. The functions.php contains the code if ( ! isset( $content_width ) ) $content_width = 620; so its not in the css.

How do I apply a code to change the width for just the singe-post?

Here are the single-post.php (in case its needed aswell)

<?php
/**
 * @package WordPress
 * @subpackage CleanMag Theme
 */
?>
<?php get_header(); ?>
<div class="grid_17 alpha">
    <div class="entry clearfix">
		<?php if (have_posts()) : while (have_posts()) : the_post(); ?>
        <h1><?php the_title(); ?></h1>
            <div class="post-meta">
                <span class="meta-date"><?php the_time('j'); ?> <?php the_time('M'); ?>, <?php the_time('Y'); ?></span>
                <span class="meta-category"><?php the_category(' '); ?></span>
                <span class="meta-author"><?php the_author_posts_link(); ?></span>
                <span class="meta-permalink"><a href="<?php the_permalink(' ') ?>" title="Permalink"><?php _e('Permalink'); ?></a></span>
            </div>
            <!-- END post-meta --> 

		<?php the_content(); ?>
        <div class="clear"></div>
		<?php endwhile; ?>
		<?php endif; ?>	

        <?php wp_link_pages('before=<div id="post-page-navigation">&after=</div>'); ?>

        <div class="post-bottom">
        	<?php the_tags('<div class="post-tags">',' ','</div>'); ?>
        </div>
        <!-- END post-bottom -->

        </div>
        <!-- END entry -->

	<?php comments_template(); ?>
</div>
<!-- END grid_17 -->
<?php get_footer(); ?>