Everything is relative. You know who said that right?
While Joe is correct about the 10^9 (I would actually write it as 1.0x10^-9), that doesn't really apply to these tanks at all lol.
Since there are approximately 343 quintillion gallons of water in the ocean, we can try to reduce it by a factor of 1.0x10-9 but we still get 343 billion gallons. If you want to be scientific about it, anything less than or equal to 343,000,000,000 gallons is a nano tank.
However, by this standard a microbubble would contain 343 trillion gallons of air and a pico tank would be anything less than 343 million gallons of water.
With that said, I've always considered anything less than 55 gallons to be a nano. There is no standard. I really wish there were.