I don't think height is a big issues wit rays. I've seen plenty of rays kept in shallow tanks with a huge footprint without any problems. So I say 18" being a good minimum height on a ray only tank.
my 2ft diameter motoro ray likes to hang out on the side of the tank and glides on it. I think it needs to be tall enough to prevent it accidentally jumping across especially if tank doesn't have lids. I think at this size it can easily tilt the lid open.
Min. 24", 30" optimal....any taller is a bonus as others have said...rays like to swim up the sides of the tank therefore better have good lid as you're be surprise how powerful adult rays can be.