The mother to my Marble female was over 20 inches and my female is over 18 inches and marbles do not grow as big as normal motoro, when you see one swim up the glass you realise just how big an animal 20 inches is, it is not like a 20 inch eel, this is a heavy and robust fish. I saw an old motoro at over 26 inches and she was a monster.
When my 18inch male died he was really heavy and he was much smaller than my female., how he flipped her to breed I will never know.
A 20 inch plus ray puts out a lot of waste, that is a huge amount of ammonia dumped in to a small glass cage.