select标签中的option，如何设置它的宽度？

有5个select标签，里面各有不同数量的option标签，代码如下：
HTML code

<td class='searchbanner'>
        <select id='back_region' class="bigregions" name="back_region">
               <option value='<?php echo $_POST['back_region'];?>'><?php echo  $_POST['back_region'];?></option>
        </select>
    </td>
    <td class='searchbanner' colspan='3'>
           <select id='back_region_type' class="bigregions" name="back_region_type">
               <option value='<?php echo $_POST['back_region_type'];?>'><?php echo  $_POST['back_region_type'];?></option>
          </select>
          <select id='back_options1' name="back_region3">
          <option value='<?php echo $_POST['back_region3'];?>'><?php echo  $_POST['back_region3'];?></option>
          </select>                          
          <select id='back_options2' name="back_region4">
          <option value='<?php echo $_POST['back_region4'];?>'><?php echo  $_POST['back_region4'];?></option>
           </select>
           <select id='back_options3' name="back_region5">
           <option value='<?php echo $_POST['back_region5'];?>'><?php echo  $_POST['back_region5'];?></option>
           </select>      
        </td>

我想让select标签表示的下拉列表的宽度与当前option的内容的宽度相一致，该如何实现？

.searchbanner select{
 width:100px;
}