Don't do it. My old ray use to use every inch of my 8' tank...
I think 8' x 24" wide would be ok if aquascaped properly, but that would be a bare minimum.
My ray use to swim the sides of the tank at 90 degrees like a race track. He would go non stop around and around for a long period of time. I think he liked how smooth the acrylic was. I'm no expert but I think these guys swim tremendous distances in the wild. I saw one snorkeling once and it was there and gone in a split second, just cruising the bottom at a very fast rate. And realize you cannot put any other fish with it. Even tangs will pick at them from time to time. That is why I had to get rid of mine.