How to set dimensions of the post thumbnails (featured images)

How do I control the dimension for post thumbnails (featured images) on the homepage (blog) of my site?

Dimension for the post thumbnails need to consistently be 470px x 370px. I have tried adjusting the dimension in Settings > Media but this does not work.

My code:

     <div class="posts__post">

        <article>
             // post thumbnail in question
             <a class="posts__post--preview" href="https://wordpress.stackexchange.com/questions/239844/<?php the_permalink(); ?>"><?php the_post_thumbnail(); ?></a>
             <p class="posts__post--tag"><?php the_category('&nbsp;/&nbsp;'); ?></p>
             <h1 class="posts__post--title"><a href="https://wordpress.stackexchange.com/questions/239844/<?php the_permalink(); ?>"><?php the_title(); ?></a></h1>
             <p class="posts__post--meta"><?php echo time_ago(); ?></p>
        </article>   

     </div>

I initially imported posts via the Codex theme unit test page and enabled featured images within my function.php file.

Add this code to your theme’s functions.php file. Images are resized by WordPress when they are uploaded. A plugin such as Regenerate Thumbnails can be used to ensure that source images have been properly generated if image sizes are changed after the initial upload.

// Sets the parameters for the post-thumbnail image size
// https://codex.wordpress.org/Function_Reference/set_post_thumbnail_size
add_action( 'after_setup_theme', 'wpse239844_image_sizes' );
function wpse239844_image_sizes() {
    set_post_thumbnail_size( 470, 370, true ); // $width, $height, $crop
}