1. Open /components/com_jcomments/tpl/default/tpl_comment.php and replace:
<?php echo $comment->author; ?>
with:
<?php echo JText::_('Guest'); ?>
2. Open /components/com_jcomments/tpl/default/style.css and set any font size in line:
#comments .comment-title {color: #b01625;font: bold 1em Verdana, Arial, Sans-Serif;margin: 0;padding: 0}
(you could replace 1em with 1.5em or 16px or any other value you need)
3. No, such feature doesn't supported now.