How to add custom size featured image

I want to display a picture in my theme and customize its size. I am adding this code to functions.php but it's not working. I've seen the code in [PLAIN]https://www.wpblog.com/register-post-…nail-wordpress/[/PLAIN] tutorial.
`` `
add_image_size (single-post-thumbnail, 750, 650);
`` `

To get post thumbnail url I am adding this code

`` `
<? php
$ thumb_id = get_post_thumbnail_id ();
$ thumb_url = wp_get_attachment_image_src ($ thumb_id, 'thumbnail-size', true);
echo $ thumb_url [0];
?>
`` `