Heatsinks tend to be rated using temp per watt for dissipation.
I found a calculator that might be helpful but it doesn't give the square inches required per watt. They do list some info for dissipation of copper on a circuit board which could be a bit helpful..
HEAT SINK CALCULATORMost likely if you do a search using "heat sink calculator" you most likely will trip across one that gives square inches of material required.
On edit..
Found a real nice calculator which looks like it should give you the info you need..
ANOTHER HEATSINK CALCMore info on heatsink design..
HEATSINK DESIGN