That would be a perfect size. I would recomend covering the pond at night. I worked at a store that had an aro pond. If we didn't cover the pond they would jump out at night.
Great size for a silver aro. I wouldnt suggest keeping one in any less than 200-250 US gallons. Bigspizz is right in that a 7x5x2 would be better. Think of it as with stingrays, you want as wide of a tank as possible, and height isn't very important at all compared to length and width.