How about the most reputable source on fish on the internet -
http://www.fishbase.org/
Potamotrygon hystrix max out at almost 16". Of course, they usually do not reach that size, but you have to take it into account. You also have to take into account that rays are delicate and are very prone to health issues with unstable pH, but I'm sure you've already researched all that.
They are for experts, not beginners.