i just posted a thread on this. i am setting up a 300 gal rubber maid, but i am also having a custom 480 gal acrylic tank built or a large custom pond. its been a hard thing for me to decide on. Miles is right on the bigger the wider the better thats for sure.
I'm having a 6 X 2 1/2 built for my Retric, should be more than enough room.
I think we need to find out what kind of ray the OP is thinking off, what you suggest would be great for the larger ones, but a Retric or Scobina would do fine in a tank smaller than 8X4, matter of fact I see that 180s (6 X 2) have been recomended for the smaller rays.
All that aside, yes, a round tank is better, but not everyone has the space for an indoor pond.
actually 8x2.5 would be better for the smaller rays and a 8x4 is not even big enough for rays like flowers tigers and leos wich get 30"+ go for the most amount of space you can get not what the minimum you think is fine...