Like on my forums? http://www.rayd.org/forums/
If so, I just searched through the code for the search form and found the function “search_form()” so I did this:
<h2><?php _e('Search'); ?></h2>
<?php search_form(); ?>
Then you just take “search_form.php” out of your bb-templates folder, drop it in the my-templates folder and change the sizes to whatever you want.