Depends on current and voltage drop.
P=U*I
where
U = voltage drop on control element (MOSFET or resistor)
I = current.
In your case, the dissipation will be ~6W, quite a lot - you definitely need heatsink to prevent MOSFET thermal destruction.
However, even simple resistor (capable of required dissipation, say 10W) will do, no need for MOSFET. It will get quite hot, though.
But, I am not quite clear what you really want to do here. Is it just a general question or you have some application in mind?
If you want regulated supply voltage, while the current is limited in case of short circuit, then you should use some of dedicated voltage regulator IC's (7812, for example) with built in current limiting.
The better way (lower dissipation, even when regulator is shorted at output) is to use SMPS IC's (
http://www.maximintegrated.com/produ...ng-regulators/) but for this approach, you have to have some experience with electronics... and the circuit will be more complicated.
http://www.jaycar.com.au/images_uploaded/LM2576.PDF
http://www.dimensionengineering.com/...ing-regulators
http://www.analog.com/static/importe...egulator_E.pdf