Ah, I just assumed you were using the Page Header add-on.
GP 126.96.36.199 is already being reviewed on WP.org, but this filter will be added in 188.8.131.52.
To make the change now, open template-tags.php in the /inc/ folder.
<?php the_post_thumbnail( 'full', array('itemprop' => 'image') ); ?>
<?php the_post_thumbnail( apply_filters( 'generate_page_header_default_size', 'full' ), array('itemprop' => 'image') ); ?>
If you do the change now, you’ll have to do it again after the next release.
Then it will be in the core files for the version after that.