Change custom featured image size in twentythirteen child theme

The function that handles thumbnail size is already hooked with after_setup_theme in \twentythirteen\functions.php.

There is no use adding a if (!function_exists()) in child functions.php because the child functions.php is always loaded before. So this would be more logical to find this in the parent file.

The answer would be to set a different prority to your child function like :

add_action('after_setup_theme', 'watercolor_setup', 11);

But as you mentioned this does not work in your case. This makes me to perplex because it should override size. I guess you’ll have to code it in each template you want to modify in your child theme:

the_post_thumbnail(array(604,310));