Use wp_get_attachment_metadata()
.
Using your example:
$img_data = wp_get_attachment_metadata( $attachment_id );
$w = $img_data['width'];
$h = $img_data['height'];
if ($w > $h) {
$class = "col-sm-12";
} else {
$class = "col-sm-6";
}
Though, to get the attachment ID, you’ll either have to put a filter that acts on every image or create a loop that uses featured images.