<select?id="test">
????<option?value="1">1</option>
????<option?value="2">2</option>
????<option?value="3">3</option></select>

var?select?=?document.getElementById('test')

select.onfocus?=?function()?{????var?options?=?this.options;????this.setAttribute("size",?options.length);
}
select.onblur?=?function()?{????this.setAttribute("size",?0);
}