Open file /components/com_jcomments/tpl/default/style.css and edit these styles:
.jcomments-links {margin-top: 10px; clear:both; text-align: left}
.comment-link, .readmore-link{padding: 0 5px 5px 14px;background-image: url(images/jc_blog.gif);background-repeat: no-repeat}
.comment-link {background-position: 0 -27px}
.readmore-link {background-position: 0 2px}
You can set (for example) font size 14px for Read comments link by changing:
.readmore-link {background-position: 0 2px}
to
.readmore-link {background-position: 0 2px; font-size: 14px;}