How to customize what excerpt shows?

Ok I managed to make categories show only excerpts and not full posts. Right now, however, they are showing only titles, meaning the excerpt is equal to 0 lines of the full post.

How can I control what is included in the excerpt?

Below you can find the php code the I changed to apply the excerpt to the categories, it was inside archive.php
_________________

<div class=”entry”>

<h3>” title=”Permalink”><?php the_title(); ?></h3>

<?php ($post->post_excerpt != “”)? the_excerpt() : BX_shift_down_headlines($post->post_excerpt); ?>

<p class=”info”><?php if ($post->post_excerpt != “”) { ?>” class=”more”>Continue Reading<?php } ?>

<em class=”date”><?php the_time(‘F jS, Y’) ?><!– at <?php the_time(‘h:ia’) ?>–>
<!–<em class=”author”><?php the_author(); ?>–>
<?php edit_post_link(‘Edit’,'<span class=”editlink”>’,'</span>’); ?>

</div>