I think the problem is that the diameter is only part of the story depotfish: they need space for that tail too which can be as long as the diameter total. I.e. a 14" diameter ray might have a tail of 12-14"...
On top of that there's an issue with lacking a gas bladder or something which means once they're moving they need space to move. That's from memory: you may want to check it out!
And, the smallest rays are actually: Potamotrygon hystrix, Potamotrygon scobinae, and the reticulata I already mentioned.
They aren't the easiest of fish to accommodate. I've seen some great articles on them before: people usually opt for round pond-like housing for them instead of the standard rectangular (or bowfront) tanks. That might be something you could look into?
Also, where are you from? The US? There are some state restrictions I've read about for: Arizona, Arkansas, California, Florida, Georgia, Hawaii, Nevada, Oklahoma, Texas and Utah.