It's not collected in one area and the areas that are polluted are so large that we can't collect enough samples to say for certain how large they are. Estimates range from 700,000 square kilometres (270,000 sq mi) to more than 15,000,000 square kilometres (5,800,000 sq mi).
Even if you could determine where it all was, filtering is not an option. It's not like mowing a lawn. Water moves. You can't filter one patch at a time. Even if you could, you'd be removing all of the plankton in the area and food chains tend to collapse when the lowest level organisms disappear. Making a dead zone in the ocean that's somewhere between the size of Texas and the twice the size of the United States would be... problematic.