<form action="#">
<p>
<select id="selF2" multiple="multiple">
<option value="0">アイテム0</option>
<option value="1">アイテム1</option>
<option value="2">アイテム2</option>
<option value="3">アイテム3</option>
</select>
</p>
<p><input type="button" value="アイテム数取得(DOM)" onclick="fGetSelectLengthDOM()" /></p>
<p id="resGetSelectLengthDOM"></p>
</form>
<script type="text/javascript">
function fGetSelectLengthDOM(){
var s="";
/* lengthプロパティ使用 */
s+="length = "+document.getElementById("selF2").length+"<br \/>";
/* options[]コレクションのlength参照 */
s+="options['length'] = "+document.getElementById("selF2").options["length"]+"<br \/>";
document.getElementById("resGetSelectLengthDOM").innerHTML=s;
}
</script>