[Amps] 'heat spreader' sizing ??

[Dan Sawyer]

The project is to mount a 1KW, 2 transistor, board on a heat sink. The 
transistor centers are about 2 inches. The question is: What should the 
dimensions of the heat spreader be? I have seen dimensions of 1/4 x 2 x 
4 up to 1/2 x 4 x 7; that represents quite a range. The questions are:

1. What thickness is required. Is 1/4 sufficient?

2. What is the minimum effective distance from the edge of the devices? 
1/2 inch? 1 inch?

Thanks,

Dan