remove custom image size set in theme_setup for featured images

It took me ages to track down why the featured image in all the blog posts were scaled down to 270px wide.

I want them to show at up to max size in the posts when the device used allows.

In found the cause in Twenty thirteen’s ‘functions.php’ in the function ‘function twentythirteen_setup()’ Twenty Thirteen sets the blog posts to a set width and height.
/*
* This theme uses a custom image size for featured images, displayed on
* “standard” posts and pages.
*/
add_theme_support( ‘post-thumbnails’ );
set_post_thumbnail_size( 604, 270, true );

I can get the result I want by changing the numerical sizes to auto. But I have to do it in Twenty Thirteen’s functions.php file. I know that this is not good practice.

I’m not very skilled with PHP. Can someone please advise me on how to overide this in the child theme I’ve developed. So far everything else I’m doing there is going OK. I’m sorry the issue is not available to view on the web. I’m running a test site locally on my computer.