According to this article it's 8 million gallons in total. That would fill a cube 31.5 meters on each side.
The water will contain about 190 becquerels of tritium per litre, below the World Health Organization drinking water limit of 10,000 becquerels per litre. That's before it's diluted. So you could drink it straight from the tap, before it goes into the ocean at all, and you still won't be exposed to higher amounts of radiation than is considered "acceptable."
Since dumping it into the ocean isn't going to make it more concentrated, the area of ocean water that will contain higher than acceptable amounts of radiation is exactly zero.