<form name="frmF" action="#">
<p>
<select name="selF" 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" onclick="fGetSelectLength()" /></p>
</form>
<script type="text/javascript">
function fGetSelectLength(){
alert("length = "+document.forms["frmF"].elements["selF"].length);
}
</script>