jquery take on this: https://gist.github.com/428898/e882078f10a06e55fa905a89a6ae65f30331746a

gistfile1.js

var setTextMeasure = function (contentElement, targetMeasure, maxSize, minSize) {

  if (!contentElement) contentElement = $(document.createElement('p'));
  if (!targetMeasure) targetMeasure = 66;
  if (!maxSize) maxSize = 16;
  if (!minSize) minSize = 9;
  
  var body = $('body');
  var sizer = contentElement.clone();

  sizer.css({'margin': 0, 'padding': 0, 'color': 'transparent', 'backgroundColor': 'transparent', 'position': 'absolute'});

  var letters = 'aaaaaaaabbcccddddeeeeeeeeeeeeeffgghhhhhhiiiiiiijkllllmmnnnnnnnooooooooppqrrrrrrsssssstttttttttuuuvwxyyz';
  sizer.text(letters);

  body.append(sizer);
  var characterWidth = sizer.width() / letters.length;
  sizer.remove();

  var contentWidth = contentElement.width();
  
  var measuredFontSize = parseFloat( body.css('font-size').replace('px', '') );

  var actualMeasure = contentWidth / characterWidth;
  var newMeasure = measuredFontSize * actualMeasure / targetMeasure;

  if (newMeasure > maxSize) newMeasure = maxSize;
  if (newMeasure < minSize) newMeasure = minSize;

  body.css('font-size', newMeasure + "px");
}