Here's the only reliable looking page I could fine : http://www.hartrao.ac.za/other/vredefort/vredefort.html

Original impact crater estimated to be 300Km in diameter. They think 70 cubic KM of soil was vaporized. For comparison's sake Mt St Helens explosion vaporized 1 cubic KM of soil and rock. Chixculub crater is 150Km in diameter, much much newer, but completely buried.