Retina Displays, Adaptive Images and WP Thumbnail Sizes

I’m setting up my thumbnails for WordPress (we have *many*!) and using adpative-images.php. Script is working great.

My Question:

If I want a 200×200 image that also looks lovely on retina, should I…

A) add_image_size( ‘homepage-thumb’, 400, 400 );

or

B) add_image_size( ‘homepage-thumb’, 200, 200 );

or

C) That other thing I’m not thinking of.

Seems like I should use A for images to scale properly on retina. In my template, I call <?php the_post_thumbnail( 'homepage-thumb' ); ?> but that’ll output a 400×400 image on non-retina!

Conceptually stumped. Ideas anyone?

http://wordpress.org/plugins/new-nine-adaptive-images/