Going to Farnell's heatsink search page and entering your dimensions and "TO220" as search parameters, I can't find the exact model, but there are bunches with thermal resistances in the order of 20-25C/Watt. Yours is a reasonable quality aluminium extrusion so better than some; I would use 20C/W as a reasonable figure unless you can find the actual data.
Which means at 7.5W from yesterday's question ... 150C expected temperature rise...
The other option is airflow, of course...
Theoretical approach: you could re-run those numbers (get a datasheet from a proper heatsink!) and see what airflow would make it work, and if a small fan (another datasheet) could push air that fast.
Experimental approach : attach a thermocouple (I got one with a £10/$15 multimeter - no longer sold but it came from Maplin) to the heatsink, and measure the temperature as you draw more current. (Start at 0.25A on one reg, given your other question. Point a CPU fan at it, add ducts, re-measure temperature. Repeat until satisfactory.
These things actually exist.
http://www.ebay.com/bhp/mosfet-heatsink
And well, roughly you could say that the amount of surface area will allow better cooling.
In combination with the amount of air flowing through/against it.
So that it will have the most possible contact with cool air.
I'm not sure what your implementation is. But I'd suggest to try passive cooling.
It really helps a lot, won't cause any noise or 'waste' energy.
(Cooling with a fan might increase the current over the mosfet and thus backfire you?)
Also, if it gets too hot for passive cooling (probably) something is wrong with your implementation.
So yes, it will work, it's effectiveness will vary on the exact implementation. But it 'should' not be neccesary to use active cooling (with a fan). The mosfet heatsinks on ebay look pretty promising and are way easier to implement and cheaper than an extra fan+temperature sensor.
Best Answer
First you need to determine how much power is dissipated in the transistor. The second factor is the maximum ambient temperature that you expect the transistor to experience. Finally, decide what maximum temperature you want at the transistor chip itself...maybe 75°C is a good target. Subtract the ambient temperature from the maximum transistor die temperature and you have the maximum temperature increase from the transistor chip to the outer surface of the heatsink. Divide the temperature difference by the power dissipated and you have a value for thermal resistance in °C/W (degrees C per watt). This is the maximum thermal resistance you can allow from the transistor die to the ambient air. Find out the thermal resistance of the transistor package itself. For the transistor you list above, this is given in the data sheet as "Thermal Resistance, Junction to Case" and it has a value of 6.25°C/W. Now subtract the thermal resistance of the transistor package from the total allowable thermal resistance you calculated earlier. Now go to the catalogs and look for a heatsink that fits the TO-220 package and has a thermal resistance, in still air, equal to or less than this value.