Other than a higher ceiling, my listening room dimensions are quite similar to yours. I've used MMGs (with subs) across the long wall with superb results. As Ralph notes, they'd probably be fine on the short wall, too (given the response null to the side), but my room isn't laid out that way. Without the subs, bottom end response is too limited for my taste, but the room size is almost ideal for the speakers, IMO.
FWIW.
FWIW.