check for thumb and thumb size on post save (wordpress)

<?php
function check_for_thumb($post){
    global $post;
    $post_thumb = get_post_meta($post->ID, '_thumbnail_id', true);
    /* GIVE ERROR NOTICE IF POST HAS NO THUMB  ON SAVE */
    if(isset($_GET['post']) && $_GET['action'] === 'edit' && empty($post_thumb)) {
        echo "<div class='error'><p>Posten saknar utvald bild</p></div>"; // message
    }
    if(!is_null($post_thumb)) $thumb_meta = wp_get_attachment_metadata($post_thumb, true);
    if(isset($_GET['post']) && $_GET['action'] === 'edit' && !empty($post_thumb)) {
    /* GIVE ERROR NOTICE IF THUMB (FEATURED IMAGED) DOESN'T MEET REQUIRED WIDTH/HEIGHT ON SAVE */
        $width = $thumb_meta['width'];
        $height = $thumb_meta['height'];
        if($width < '485' || $height < '360') {
            echo "<div class='error'><p>Utvald bild bör vara minst <strong>485 x 360 px</strong>. (Nuvarande bild är $width x $height px.)</p></div>";
        }
    }
}
add_action('admin_notices', 'check_for_thumb');
?>