jQuery plugin
Plugin can resize textarea field when user type some text
width - width of hidden container, must be equal to textarea width (default: outerWidth)
height - minimum height of hidden container (default: outerHeight)
refresh - update height of textarea
<script type="text/javascript" src="/path/to/textarea-autoresize-js.js"></script>
<script type="text/javascript">
$('textarea').autoresize({
width: 400,
height: 200
});
</script>
$('textarea').autoresize('refresh');