Here is the steps:

• Go to wordpress admin panel --> Design tab --> Theme Editor.
• Select Main Index template (index.php) to edit it
• Find the line  "<?php the_content('Read the rest of this entry &raquo;'); ?>"
• And replace it with the following code.

<?php

$content = get_the_content($more_link_text, $stripteaser, $more_file);
if(strlen($content)>500) // chanage 500 as per your needs
{
$part_msg=substr($content, 0, 500);// chanage 500 as per your needs
echo '<p>'.$part_msg.'... ';
?>
<br /><br />
<a href="<? comments_link(); ?>">Read More...</a></div>
<?php
echo '</p>';
}
else
{
echo '<p>'.$content.'</p></div>';
}

?>

Where "500" represents number of characters that you want to show.

If you cannot able to make the above changes with the wordpress theme editor, use your ftp.

Here is the template directory

/public_html/blog/wp-content/themes

or

/www/blog/wp-content/themes

where blog is the directory where i installed my wordpress