Changing the thumbnail size
If you want to change the size of thumbnails that can be displayed next to the related posts, use the filter ‘rp4wp_thumbnail_size’.
You can use below code snippet to change the Related Posts for WordPress thumbnail size.
Please note that above filter has to return a valid WordPress thumbnail size. You can read how to add thumbnail sizes to WordPress here.
After you’ve added the new thumbnail size you have to regenerate your thumbnails, this is WordPress default behavior. There ‘s a plugin available that helps you in this.