try use javascript in comment_form.html template.
for example: insert into <textarea> tag (not between <textarea> and </textarea> !) this:onkeyup="textrem(this,this.form.rem);"; onkeydown="textrem(this,this.form.rem);"; onkeypress="textrem(this,this.form.rem);"; onblur="textrem(this,this.form.rem);"; onfocus="textrem(this,this.form.rem);"; onchange="textrem(this,this.form.rem);"
Then, after </textarea> insert:<br /><input type="text" name="rem" class="commentinput" style="width:30px"; maxlength="3" size="3" value="255" onblur="textrem(this.form.comment_text,this);" readonly /> characters remaining
<script language="javascript">
var maxlimit = 255
function textrem( field, rem ) {
if ( field.value.length > maxlimit )
{
field.value = field.value.substring( 0, maxlimit );
return false;
}
else
{
rem.value = maxlimit - field.value.length;
}
}
</script>