That is a valid image size, there's a bug at the moment which is causing the error message but it is in the process of being looked into.
Seeing the world with a 3:2 aspect ratio...
My images on Flickr
The 600 pixel height falls below our minimum size. 16:9 at 1024 pixels wide does not work.
Remember that you can now go up to 1280 Pixels wide if you wish. 1280x720 is the size for 16:9 format.
If it 'ain't broken........ Don't try to mend it !