Depending how many manufacturers you have even a drop down box may not be the answer. We use a scroll box instead. Here is our menu_manufacturers.tpl (v4.1.10 but should work for you also). It goes in skin1/modules/Manufacturers/menu_manufacturers.tpl
Code:
{* $Id: menu_manufacturers.tpl,v 1.1.2.7 2008/04/17 05:45:57 max Exp $ *}
{if $manufacturers_menu ne ''}
{capture name=menu}
<form name="form_manuf" method="get" action="manufacturers.php">
<select size="12" name="manufacturerid" onchange="document.form_manuf.submit()">
{section name=mid loop=$manufacturers_menu}
<option value="{$manufacturers_menu[mid].manufacturerid}">{$manufacturers_menu[mid].manufacturer|truncate:18:"...":true}</option>
{/section}
</select>
</form>
{/capture}
{include file="menu.tpl" dingbats="dingbats_categorie.gif" menu_title=$lng.lbl_manufacturers menu_content=$smarty.capture.menu}
{/if}
Changing the number in "select size=" will adjust how many manufacturers are displayed at one time, and "truncate:18:"...":true" will adjust the length of the manufacturers name displayed.