He's saying 8 feet long, by 3 feet tall, by 3 feet wide.
I don't know the math, but that's well over 200 gallons. I would say, as a basic rule of thumb, look into a tank over 200 gallons. Once you see how much they cost in your area, look into ones that are wider rather than taller. Arowanas are a top water fish, so they'll stay in the top half of your tank, so depth isn't as important as width. The wider, the easier it is for them to turn around. They turn a lot, pretty much all the do with the majority of their time is pace back and forth in your tank.