Big female rays would be better served in a large pond or some sort of very large tank. They can get quite large disc wise and long as well. Searching this site will answer your questions better than this thread will! Good luck
a 6' L X 3' W would be good starting footprint for many freshwater stingrays.. it all depends on what type of stringray of course, and how many you plan to keep
For any other ray apart for hystrix scobina and retic you need a min tank size of 7ft long x 4ft wide
Someone above said 30 inch wide that will last you 1.5-2 years max for most rays