For pond, bigger is better with snappers. However, AST's don't tend to roam as much as you'd expect them to, so keep that in mind.Awsome so what size do you recommend?
Once the turtle starts to get too big for say, a 125 gallon aquarium, I'd say you should upgrade to your idea of the 500 gallon pond. From there you'll have to see how much bigger he/she gets, from my understanding there's a wide range of sizes they max out at, and just when you think they can't get any bigger, they find a bigger one.