una cosa del genere?

codice:
<html>
<head>

<style type="text/css">

.short {
 width:50px;
}

.long {
 width:100px;
}

</style>

<script language="JavaScript">

function f(indice) {
 var e=document.getElementById('menu');
 if(indice==0) e.className='short'; else e.className='long';
}

</script>
     
</head>

<body>

<select id="menu" class="short" onchange="f(this.selectedIndex);">
<option value="v1">Valore1</option>
<option value="v2">Valore2Valore2Valore2</option>
</select>

</body>
</html>
ciao