[@robkk Moderator 11 years ago]

This is going to increase the font-size of the topic and reply content of the same exact specified font size on your site.

Add this custom CSS to your child themes style.css file or anywhere else you can put custom CSS like a custom CSS plugin.

.bbp-topic-content p,
.bbp-reply-content p {
    font-size: 1rem;
}

If you want to increase the font size of other areas of the forum be more specific.