How to change the featured image / post thumbnail size on category / archive pages?

I am trying to figure out how I can change the size of the featured images on category and archive pages on my WordPress website using a custom theme.

In my functions.php file, I have added the following code:

/** Enable support for Post Thumbnails on posts and pages. */

/** Featured image thumbnail size. */
function pdb_post_thumbnails() {
    set_post_thumbnail_size(450, 450);

add_action('after_setup_theme', 'pdb_post_thumbnails');

I’ve saved, uploaded the file and then regenerated the images uploaded but when I visit /category/uncategorised/ all of the featured images are the full, original size and not 450 x 450.

What am I doing wrong or missing?