Ok thanks!
I found this code suggestion, really simple to apply, I paste it below in case someone else need it.
insert this at the top of vote/ form.php view
<script>
function limitText(limitField, limitNum) {
if (limitField.value.length > limitNum) {
limitField.value = limitField.value.substring(0, limitNum);
}
}
</script>
insert this into the textarea (change 500 to your wanted maximum characters)
onKeyDown="limitText(this,500);" onKeyUp="limitText(this,500);"
the whole textarea should look like this
<textarea type="text" name="hikashop_vote_comment" id="hikashop_vote_comment" class="hikashop_comment_textarea" placeholder="<?php echo JText::_('HIKASHOP_POST_COMMENT');?>" onKeyDown="limitText(this,500);"
onKeyUp="limitText(this,500);"></textarea>