the volume of the dam according to wikipedia is 27 million m^3. if that was to spread over an area of 27 million m^2, it would be 1 meter deep.  27 million m^2 is around 27 km^2, or roughly a square of 5 by 5 kilometres.  of course, because of the flow of water, it could damage larger area, but i doubt it could flood a third of china.
Bulgaria