To answer your question, I've had them 24" across.
The more in depth reply is that in the hobby, there isn't a FL Freshwater Stingray. There are marine rays that live short periods in FW, and will end up dying unless you take them to full salt water.
Even while in FW, you're looking at over 10 times the ammonia production..... so you are looking at thousands of gallons to offset that.
It's doable - sure, but it's just not realistic.