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