Set auto width - jQuery Autocomplete

I have ever done this using a parameter openthat starts after the menu opens.

$('#'+pmId).autocomplete('mst_get_masterinfo.php?vValid=15&vRetType=1&vCategory='+vCatCode,{
    delay:10,
    cacheLength: 0,
    mustMatch:true,
    open: function() { 
        // After menu has been opened, set width to 100px
        $('.ui-menu')
            .width(100);
    }  
});