By definition, a "center" is a single point. No matter how big something is, the center is a point. It's the place where there's as much ground motion on one side as on the other, both north and south, as well as east and west. The epicenter is the point on the earth's surface which is directly above the actual center, which is the point where there's also the same amount of motion above it as below it, as well as in the other two dimensions.
You are correct. The thrust Earthquake covered a distance of somewhere in the neighborhood of 700 miles. 1200 Km was the figure I heard, and my conversions suck. Average uplift was about 15 Meters. that is just under 50 feet. So part of the sea floor rose by 50 feet, while parts to the west SUNK as the Indian Plate subducted under the Burma plate. Hense the huge waves.