Males tend to stay smaller and the smallest rays are scobina, but they are so expensive it would probably be cheaper to buy a bigger tank and a different ray.
For a while you could do hystrix or reticulata (reticulated).
If you do something larger like a motoro you will need a MUCH bigger tank, but if you do some smaller ones (less than a foot or 14 inches in disc diameter) then you would only need about a 6' x 2.5' tank.