function Autosize(txt:TextField):void
{
  //You set this according to your TextField's dimensions
  var maxTextWidth:int = 145;
  var maxTextHeight:int = 30;

  var f:TextFormat = txt.getTextFormat();

  //decrease font size until the text fits
  while (txt.textWidth > maxTextWidth || txt.textHeight > maxTextHeight) {
    f.size = int(f.size) - 1;
    txt.setTextFormat(f);
  }

}