Theme currently showing thumbnails, how to show full size?

Here is the relevant code from home.php (the Blog Posts Index):

<?php 
if($grisaillePostCount == 1) { 
    the_post_thumbnail(); 
} else { 
    the_post_thumbnail('following-post-thumbnails'); 
} 
?>

Here is the relevant code from index.php:

<?php the_post_thumbnail('following-post-thumbnails'); ?>

So, the difference is the default thumbnail size, and the custom 'following-post-thumbnails' image size.

In functions.php, the Theme defines the defualt 'thumbnail' size as:

set_post_thumbnail_size( 590, 275, true ); // 590 pixels wide by 275 pixels tall, hard crop mode

…and sets the 'following-post-thumbnails' size as:

add_image_size( 'following-post-thumbnails', 250, 200, true ); // 250 pixels wide by 200 pixels tall, hard crop mode

So, to see the default 'thumbnail' image size in the default template (index.php), change this:

<?php the_post_thumbnail('following-post-thumbnails'); ?>

…to this:

<?php the_post_thumbnail(); ?>

Edit

And to display the 'full' image size, use this:

<?php the_post_thumbnail( 'full' ); ?>