Code:
<select name="listCategories" onchange="window.location=this.options[selectedIndex].value" style="width:100%">
<option value=""><font class="VertMenuItems">Select A Product Category</font></option>
{foreach from=$categories_menu_list item=c name=categories}
{if $c.order_by < 899 && $c.product_count ne 0}
<option value="home.php?cat={$c.categoryid}">{$c.category} </font></option>
{/if}
{/foreach}
</select>
You can adjust the % value or use a fixed pixel value -
style="width:200px"