[@rescuethemes Participant 12 years, 3 months ago]

I was able to locate bbp_single_user_details_avatar_size in bbpress/templates/default/bbpress/user-details.php which eventually lead me to the following function that accomplishes what I needed (resizing the avatar):

function my_bbp_change_avatar_size($author_avatar, $topic_id, $size) {
    $author_avatar = '';
    if ($size == 14) {
        $size = 24;
    }
    if ($size == 80) {
        $size = 110;
    }
    $topic_id = bbp_get_topic_id( $topic_id );
    if ( !empty( $topic_id ) ) {
        if ( !bbp_is_topic_anonymous( $topic_id ) ) {
            $author_avatar = get_avatar( bbp_get_topic_author_id( $topic_id ), $size );
        } else {
            $author_avatar = get_avatar( get_post_meta( $topic_id, '_bbp_anonymous_email', true ), $size );
        }
    }
    return $author_avatar;
}

/* Add priority (default=10) and number of arguments */
add_filter('bbp_get_topic_author_avatar', 'my_bbp_change_avatar_size', 20, 3);
add_filter('bbp_get_reply_author_avatar', 'my_bbp_change_avatar_size', 20, 3);
add_filter('bbp_get_current_user_avatar', 'my_bbp_change_avatar_size', 20, 3);

Increase avatar size in bbpress