For those wanting a simple solution without jQuery, here's a way.
textarea and message container to put in your form:
<textarea onKeyUp="count_it()" id="text" name="text"></textarea>
Length <span id="counter"></span>
JavaScript:
<script>
function count_it() {
document.getElementById('counter').innerHTML = document.getElementById('text').value.length;
}
count_it();
</script>
The script counts the characters initially and then for every keystroke and puts the number in the counter span.
Martin