Script fra jsfiddle.net

<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>demo</title>

<style type="text/css">
.box {
  background: yellow;
  margin: 10px;
  text-align: center;
}
</style>
<script type="text/javascript"
    src="http://ajax.googleapis.com/ajax/libs/jquery/1/jquery.min.js"></script>
<script type="text/javascript">
    $(document).ready(function() {
$('.box').each(function(i, box) {

  var width = $(box).width(),
    html = '<span style="white-space:nowrap">',
    line = $(box).wrapInner(html).children()[0],
    n = 100;

  $(box).css('font-size', n);

  while ($(line).width() > width) {
    $(box).css('font-size', --n);
  }

  $(box).text($(line).text());

});
    });
</script>
</head>
<body>
<div class="box" style="width:700px">This is a sentence</div>
<div class="box" style="width:600px">This is a sentence</div>
<div class="box" style="width:500px">This is a sentence</div>
<div class="box" style="width:400px">This is a sentence</div>
</body>
</html>