W sumie to można kombinować z JS coś typu
Kod
<form name="formname">
<select name="selectname" size="1" onmouseover="rozwin()" onmouseout="zwin()" onchange="zwin()">
..
..
<script language="JavaScript">
function rozwin(){
formname.selectname.size = 10;
}
function zwin(){
formname.selectname.size = 1;
}
</script>