Post length

Hello. I am developing a custom WordPress theme and I wondered if there’s a way to limit the post length when displaying it on the index page? I have read all about the_content() and the_excerpt(), but none of them seem to work. The code I am using in my page.php template is:

<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); ?>

 <h2><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h2>

   <?php the_content(); ?>

 <?php endwhile; else: ?>

 <?php endif; ?>

And to get the content from my pages and display it in a custom css box on my index page, I use this code:

<?php
$page_id = 78;
$page_data = get_page( $page_id ); 

echo apply_filters('the_content', $page_data->post_content);
?>

The problem is that the whole content of the page is displayed in this box, and I want to reduce it. I have tried using the ‘More’ option in the WYSIWYG editor, but still no result. Does anyone have any suggestions on how to fix this? Thank you