font-resize.js

// an element with class 'font_resize' with css properties: 
//     * max-height
//     * max-width
//     * font-size
// the element has children span elements, the font size inside these spans will re resized.

;(function($){
  $.fn.fontResize = function(options){
    var spans = $('span:visible', this);
    var heightMax = parseInt($(this).css('max-height'), 10),
        widthMax = parseInt($(this).css('max-width'), 10),
        fontSize = parseInt($(this).css('font-size'), 10); /* max font size */

    do{
      var textHeight = 0,
          textWidth = 0;

      $(spans).each(function(){
        $(this).css('font-size', fontSize);
        if($(this).height() > textHeight){ textHeight = $(this).height(); }
        textWidth = textWidth + $(this).width();
      });
      fontSize = fontSize - 1;
    } while ((textHeight > heightMax || textWidth > widthMax) && fontSize > 3);
    return this;
  };
})(jQuery);

$(document).ready(function(){
  $('.font-resize').fontResize();
});