I have debated if my 240 at 8*2*2 is big enough for one. I would do the display open with maybe a pillar or two in the back corners.
Bigger the better. Maybe 75 is for QT purposes?
You should have no problem fitting one into a tank that size, and it
will sort of fit, that is, if you're talking about the ribbontail stingray. But they grow up to 28" inches long, which is definitely longer than the width of your tank, and as such I wouldn't suggest keeping one unless you want a fish to die of stress.
Remember, they want footprint, not gallonage. The footprint of your tank is 16 square feet, which might even theoretically ok if it was 4 foot long by 4 foot wide, or perhaps 5 foot long by 3 foot wide (though that's pushing it). Even the most experienced ray keepers haven't been able to keep it alive in captivity. The only reason it can fit in a relatively small tank is because (1) it's a smaller ray than most and (2) it naturally hunts on shores and hides between rocks when resting. However, overall, if you're willing to drop large amounts of money on one on the off chance that you can miraculously keep it alive, then go ahead, if you can keep it alive initially and for the first several months, I don't see why it can't live in that tank. .
Oh, so you didn't want the ribbontail? You want the slightly easier to keep blue spotted stingray? Well, that's completely out of the question for you tank. The ribbontail grows to 1' in diameter for its circular portion. The regular blue spotted, however, grows to 1' 8" in diameter, and as such needs something at least twice as wide as that.
I hope I didn't come off as harsh, but this is definitely a serious matter. Most rays will not fit in standard tanks, and if they do barely fit, they'll be stressed out by the size of their tank and die prematurely. One of these rays is semi impossible to keep, and the other tops the list as one of the most difficult to keep stingrays.
