Any way to remove or reduce the size of the “Featured Image” on a blog post?

I have checked your site and see that you are using the big image size. If you want to get the image size as our demo, you should use the image size is 770 x 440. 
In this case, you can use the following code: 

.single #main>.post .entry-header {
height: 440px;
object-fit: cover;
}
.single #main>.post .entry-header {
margin-top: 50px;
}

Or you can open the content-single.php file in your theme and find the line 21. 
Replace it with the following code: 

<div class="entry-thumbnail"><?php the_post_thumbnail('dw-focus-slider'); ?></div>