Placing the following css code in child theme's custom css OR the theme custom CSS option area to change font size.

#bbpress-forums .bbp-topic-title .bbp-topic-started-in {
font-size: 14px!important;
}

#bbpress-forums .bbp-topic-freshness .bbp-topic-meta span {
font-size: 14px !important;
}

#bbpress-forums .bbp-replies p {
font-size: 14px;
}

#bbpress-forums .bbp-topic-content strong {
font-size: 14px;
}