There's quite a bit of math to be done when you need to calculate a size for your heatsink.
As there are many options to choose from if you don't know the size yet, is it just better to pick the largest one? (assuming you have unlimited space for it);
Example
The other option would be to use a smaller one but with a fan for forced airflow like this one.
So is it more efficient if I put a small heatsink with a fan or just use a large but not 100x bigger heatsink?
Best Answer
If you have the space, and can find the right geometry of heat-sink that dissipates the amount of heat you need rid of at an acceptable cost, a plain heat-sink with no fan is always preferable.
A fan consumes power so reduces the efficiency of your system. Moreover, fans can and will eventually fail, or be blocked, at which point over-heating can kill whatever you are trying to cool if you do not have temperature sensing and some form of thermal shut-down circuitry.
Fans are also noisy.
As such, in general, fans should only be used if surface space is limited, the heat-sink would need to be extremely large and expensive, or the heat-sink can not be located somewhere that has free air-flow.
However, on occasion, a fan may be required anyway for other reasons. If that is so then it is prudent to arrange your heat-sink to take advantage of the existing forced air-flow.