@Alex: Your bbpress forum font are controlled from bbpress forum css.
So, try adjusting font size in following CSS and add it in “Appearance => Theme Options => Custom CSS” box.
@media screen and (min-width: 769px) {
/* =bbPress Style
-------------------------------------------------------------- */
#bbpress-forums {
font-size: 12px;
}
#bbpress-forums ul.bbp-lead-topic,
#bbpress-forums ul.bbp-topics,
#bbpress-forums ul.bbp-forums,
#bbpress-forums ul.bbp-replies,
#bbpress-forums ul.bbp-search-results {
font-size: 12px;
}
#bbpress-forums .bbp-forums-list li {
font-size: 11px;
}
#bbpress-forums div.bbp-forum-title h3,
#bbpress-forums div.bbp-topic-title h3,
#bbpress-forums div.bbp-reply-title h3 {
font-size: 16px;
}
#bbpress-forums div.bbp-forum-author .bbp-author-role,
#bbpress-forums div.bbp-topic-author .bbp-author-role,
#bbpress-forums div.bbp-reply-author .bbp-author-role {
font-size: 11px;
}
span.bbp-author-ip {
font-size: 11px;
}
/* =Breadcrumb and Tags
-------------------------------------------------------------- */
div.bbp-breadcrumb,
div.bbp-topic-tags {
font-size: 12px;
}
}