​x
 
<html>
<head>
<script type="text/javascript">
function changeSize()
  {
  document.getElementById("mySelect").size=4
  }
</script>
</head>
<body>
​
<form>
<select id="mySelect">
  <option>苹果</option>
  <option>桃子</option>
  <option>香蕉</option>
  <option>桔子</option>
</select>
<input type="button" onclick="changeSize()" value="改变大小">
</form>
​
</body>
</html>
​