Actually the most commonly seen rays which are Potamotrygon reticulata and Potamotrygon hysterix will both attain a disc size of 16" if cared for properly, the other commonly seen species such as Potamotrygon motoro and Potamotrygon leopoldi will get to around 20".
As said the minimum sized tank you should be looking at to house a ray for life is a 5x2x2 and prefferably bigger with a long wide tank being required rather than a tall one.
As for tankmates there are many combinations you could try, they work well with most non aggressive medium-large fish. Fish to avoid are large Characins, aggressive Cichlids and sucker mouthed catfishes (plecs etc).