Single Post Template with 2 featured image options.

single.php

<?php
/**
 * The template for displaying all single posts.
 *
 * @package theme_name
 */

get_header(); ?>

<!-- Option #1 - Default way of calling the "Full" Featured image size and placing it inside a div that would be 100% wide on screen. -->
  <?php if ( has_post_thumbnail() ): ?>
     <div class="full-width-div">
       <?php the_post_thumbnail('full'); ?>
     </div>
  <?php endif; ?>
<!-- END Option #1 -->

<!-- Option #2 - Get post thumbnail image src and apply it as a css background image to the div. This requires a height/min-height on the div if it is empty. -->
  <?php if (has_post_thumbnail( $post->ID ) ): ?>
      <?php $image = wp_get_attachment_image_src( get_post_thumbnail_id( $post->ID ), 'single-post-image' ); ?>
      <div class="full-width-div" style="min-height: 200px; background-image: url('<?php echo $image[0]; ?>')"></div>
  <?php endif; ?>
<!-- END Option #2 -->

  <div id="primary" class="content-area">

    <main id="main" class="site-main" role="main">

    <?php while ( have_posts() ) : the_post(); ?>

      <?php get_template_part( 'template-parts/content', 'single' ); ?>

      <?php the_post_navigation(); ?>

      <?php
        // If comments are open or we have at least one comment, load up the comment template.
        if ( comments_open() || get_comments_number() ) :
          comments_template();
        endif;
      ?>

    <?php endwhile; // End of the loop. ?>

    </main><!-- #main -->
  </div><!-- #primary -->

<?php get_sidebar(); ?>
<?php get_footer(); ?>