Increase dropdownlist width according to selected value.

I am not able to Increase dropdownlist width according to item length.
I am trying as like this..
 

<script type="text/javascript">
    function autoWidth()
    {
        var maxlength = 0;
        var mySelect = document.getElementById('Select1');
        for (var i=0; i<mySelect.options.length;i++)
        {
            if (mySelect[i].text.length > maxlength)
            {
                maxlength = mySelect[i].text.length;
            }
        }
        mySelect.style.width = maxlength * 10;
    }
</script>

 

but it is not working in mozila. Please help me...