well, basicly you can calculate like this: if you know the power disipated on the IC, lets say P=20W. then if you have a room temperature of about 20 dergees Celsius, and you allow the IC to heat up to 80 dergrees that means that you have 60 dergrees difference. that means that the temperature coeficient(i am not sure it is called like this) of the heatsink should be mminimum 60/20=3dereees/W
usually whae you buy a heatsink you can ask abuot that.
you might find sheets of aluminium with different thickness wich you can use to build your heatsinks an they say the heat conductance of it and you can calculate the surface of the heatsink.
now, back to your problem.....i dont think that you will have more than 20-30 W disipated on the heatsink so.....