resize.html

<!-- Automatically resize a textarea's height (like Facebook) -->

<textarea id="comment" rows="1" cols="50"></textarea>

<script type="text/javascript" charset="utf-8">
  function autoResize(fieldId) {
    var length = $(fieldId).value.length;
    
    if(length > 0) {
      $(fieldId).rows = Math.floor(length / $(fieldId).cols) + 1;
    } else {
      $(fieldId).rows = 1
    }
  }

  $('comment').observe('keyup', function(){autoResize('comment');});
</script>