I looked in to this recently (for a motor driver), and got conflicting information. The data sheets sometimes give a suggested area, but that is usually for a top layer pad thermally connected to an infinite internal ground plane.
The best data I could find for a top layer single sided heat sink was 1000/cm^2. Using that, and going for 75 C rise, you would need 26.67 square cm. About 2" x 2".
I have not yet been able to test my board under full load, so I can't say how reliable this information is.