Difficult post to answer.
Whilst a Motoro could live in a 4X2 for a year or so it is not just down to the size of the tank and ray in terms of swimming space.
These Guys produce a LOT of waste therefore a small body of water will fluctuate in quality far faster than a big one, the ammonia produced needs to be dealth with by BIG filters, personally I would want an FX5 plus another cannister as a minimum but sump would be better.
You need to commit to 30% water changes weekly and regularly test the water. If you slip by 1-2 weeks you will run in to trouble.
I do not know your level of fishkeeping experience so it is hard to say what you need to know.
Rays are easy to keep but you need to know how to manage the large amounts of waste. Mine feed a lot and produce vast amounts of poop.
My Nitrate was at 5ppm on Saturday following water changes, I tested it again on the following Thursday and it was already 50ppm. Mine is a 300 Gal tank with 1 medium Ray and 1 pup.