var maxWidth = 0;

//total amount of li-tags

var maxli = $('ul li').length-1;

​

$('ul li').each(

    function(index){

            //wrap li-content width inline element like <span>

        $(this).wrapInner( "<span></span>");

​

            //get max-width of spans

        if( $(this).children('span').width() > maxWidth ){

            maxWidth = $(this).children('span').width();

        }

​

        //complete

        if(index == maxli){

                    //set width of parent ul-tag to max-size of the longest li-tag

            $(this).parent('ul').width(maxWidth);

        }   

    }

);