Ahh, yes, if you're using the Image post format, you'll want a 700 x 467 image, or stay at that ratio to avoid cropping.
But that could be changed to better suit your work, adding a new image size in a functions.php file in a child theme:
function my_image_size() {
add_image_size( 'mynewsize', '900', '467', true );
}
add_action( 'after_setup_theme', 'my_image_size' );
(You can see how add_image_size works here.)
Then copying content-image.php into your child theme, changing the lines (10 and 25) that reference the thumbnail size:
$thumb = get_the_post_thumbnail( $post->ID, 'mynewsize' );
$thumb = wp_get_attachment_image( $attachment->ID, 'mynewsize', false );
You'd need to tweak the styles a bit, as the image will be constricted by the width of the content area unless you position them differently, move them outside the content area, or set a larger max-width.