BuddyPress.org

Font-size coming from this block:
#bbpress-forums ul.bbp-lead-topic,
#bbpress-forums ul.bbp-topics,
#bbpress-forums ul.bbp-forums,
#bbpress-forums ul.bbp-replies {
font-size: 12px;
}
Change that by copying and pasting the block into your style.css file and changing it to
font-size: 14px !important;
or whatever unit of measurement is used in your theme.

While you’re there, I suggest that you add this block to your stylesheet as well:
.bbp-body .bbp-topic-title img.avatar,
.bbp-body .bbp-topic-freshness img.avatar,
.bbp-template-notice img.avatar,
.bpp-author-avatar img.avatar {
width: 14px;
}