Would recommend 8ft x 3ft x 3ft to keep this ray. Besides that, you will need a massive skimmer and filtration. Hard to discuss on tank mates, but I would say not a lot of possibilities. IMO they are very easy feeders and will eat about anything meaty you can find in the sea (Squid, shrimp, ...). I would only suggest you to keep one if you have a lot of experience... To be honest, unless you have some kind of a huge tank like the public aquariums, I think these animals belong in the ocean and not in a “normal private tank”.
Besides that, they are indeed very interesting and beautiful animals!