They can quite easily get a disc diameter of 18" and require a MINIMUM of a 5x2ft tank, although most people accept that you need it to be 3ft wide for the rays to be comfortable. If I was setting up a tank for rays it would be 6x3x1ft.
There are some which get smaller, but only marginally with the smallest still being about 2ft including tail, and most people agree that they still need a large tank to be happy.
Ben