Can’t show custom post thumbnail sizes as background images

Use this. you will need to call thumbnail sizes.

$small_regular = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID), 'small_regular' );
$big_regular = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID), 'big_regular' );
echo "<style type="text/css">";
echo ".entry-featured-image { background-image: url('" . $big_regular[0] . "'); }";
echo "@media screen and (max-width:48em) { .entry-featured-image { background-image: url('" . $small_regular[0] . "'); } }";
echo "</style>";