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

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
}