Related posts image size

Put it inside the <a href=> as I believe that's its purpose for your site. :D

Should be something like this.

<div class="post-image">
   <a href="<?php echo $post_link;?>">
      <?php the_post_thumbnail();?>
   </a>
</div>

Thanks, @Elvin!

<div class="post-image">
   <a href="<?php echo $post_link;?>">
      <?php the_post_thumbnail();?>
   </a>
</div>

I added this code to the previous posts. The posts are listed one under another where they should be listed side by side.

Hi there to keep the grid - you need the article container around them:

<article class="post-333 type-post status-publish format-standard has-post-thumbnail hentry category-english-level-tests generate-columns tablet-grid-50 mobile-grid-100 grid-parent grid-33" itemtype="https://schema.org/CreativeWork" itemscope="">
<div class="post-image">
   <a href="<?php echo $post_link;?>">
      <?php the_post_thumbnail();?>
   </a>
</div>
</article>

Thank you so much to you all!

Glad we could be of help