下面举个例子吧:
<SCRIPT LANGUAGE="JavaScript">
<!--
function changeSize(obj){
document.all.modes.style.fontSize = obj.value
}
function changeFont(obj){
document.all.modes.style.fontFamily = obj.value
}
//-->
</SCRIPT>
<select name="select" onChange="changeFont(this)">
<option value="黑体">黑体</option>
<option value="宋体" selected>宋体</option>
</select>
<select name="select2" onChange="changeSize(this)">
<option value="9">五号</option>
<option value="12" selected>四号</option>
</select>
<textarea name="modes" cols="52" rows="6" style="width: 500pxheight: 300px"></textarea>
-----------------------------------------------
你应该先用style指定textarea的长和宽,不然样式变了,文本区域的大小也会随之发生变化