Featured image size in ADMIN panel?

You can filter 'admin_post_thumbnail_html' and replace the img element with a new one in the size you need.

Everything else will get messy and might lead to side effects.

• Filtering image_downsize while watching for a size of 266×266 might affect images that are not meant to be shown in the metabox.
• Changing the size for post-thumbnail temporary is not easy, because there are no safe points to start and end such a filter.