Better handling for size args

@joe - re [your comment here](https://github.com/humanmade/Custom-Meta-Boxes/pull/81#discussion_r7122139) regarding the mixed up array that was returned... I had a think about how it could be better.

The function should return an array in a saner format which is then used to generate inline styles - and we can just pass the original size arg straight to wp_get_attachment_image so nothing breaks.