you'll need to edit live-comment-preview.php, and the comment form for your theme -- probably called comments.php.
In comments.php add the code for your counter... it can be code of your choosing so long as it has an id:
word count <span id="wordcount">n/a</span>
In live-comment-preview.php add the following functions just above the event listener (about line 144) :
function initWordCount() {
if(!document.getElementById)
return false;
var cmnt = document.getElementById('comment');
var cnt = document.getElementById('wordcount');
if ( cnt && cmnt)
cmnt.onkeyup = function() {
cnt.innerHTML = numWords(this.value);
}
}
function numWords(string)
{
string = string + ' ';
string = string.replace(/^[^A-Za-z0-9]+/gi, "");
string = string.replace(/[^A-Za-z0-9]+/gi, " ");
var items = string.split(" ");
return items.length -1;
}
add an event listener (about line 184):
addEvent(window, "load", initWordCount);
This is untested, but it should work...