Actually Bat rays can get up to 6' (about 1.8 meters) in disc width.
According to "Aquarium Sharks & Rays" by Scott Michael.
Gray Smoothhounds need a minimum of a 1700-1800 gallon tank/pond. And Bat rays need minimum of a 4,800-5,000 gallon pond
And keep in mind - Mr. Michael's estimates tend to be on the low side.
A good ball park is figure for a minimum tank/pond for an adult gray smoothhound is about 2,500 gallons & for an adult Bat ray figure at least 6,000 gallons.