console 命令行工具 X clear

console

function changeSize()
  {
  document.getElementById("mySelect").size=4
  }

<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>