我有一个简单的下拉菜单：
<form class="feedmenu" align="center">
<select onchange="showRSS1(this.value)">
<option value="">Select an RSS-feed:</option>
<option value="ATO-HK">Asia Times</option>
<option value="HKH-HK">Hong Kong Herald</option>
<option value="Gov-HK">News.gov.hk</option>
<option value="RTHK-HK">Radio Television Hong Kong</option>
<option value="SCMP-HK">South China Morning Post</option>
<option value="STD-HK">The Standard</option>
</select>
</form>
如何指定下拉框打开后的大小 - size="x"属性不会执行此操作，height="x"属性也不会。