Maximum Textarea Characters Tutorial
Place this script just above the closing body tag.
<script type="text/JavaScript"> function limitText(textArea, length) { if (textArea.value.length > length) { textArea.value = textArea.value.substr(0,length); } } </script>
Place onKeyPress="limitText(this,20);" in the textarea tag so it looks like the snippit below. Change the value to suit.
<textarea onKeyPress="limitText(this,20);"></textarea>