i don't know about this. from where i come to know about heat sinks, there details, heat rise per watt and other calculations???? kindly tell me.
Have a read through the design part of this page - https://en.wikipedia.org/wiki/Heatsink
Basically when a heatsink is sold, part of its specifications are its temperature rise per watt of energy put into it. A small heatsink may have a temperature rise of 20 degrees for every watt you ask it to get rid of while large heatsinks may have a 0.1 degree per watt. This is greatly simplifying things but it will give you the general idea.