Find out the effective width of element and resize text in it
<table><tr><td id=txtcell nowrap><span id=txt>My long text is long. My long text.</span></td></tr></table>
<br>Font Size : <span id=mes></span>​

#txtcell{ max-width:200px;}

var fontSize = 20;
var reduce = function() {
    while ($('#txt').width() > 200) {
        fontSize -= 1;
        $('#txt').css('font-size', fontSize);
    }
    $('#mes').html(fontSize); // this line is only for demonstration, remove it in "production"
};
reduce();

td {width: expression(this.style.width);}