This is just the setup I had immediately I discovered the fan was so noisy when directly connected to the 12v supply. As the others have said you just have to bear in mind the power that will need to be dissipated by the resistor.

Something else to bear in mind is that the quoted current consumption of your chosen fan will be at it's designed operating voltage. As you're going to be running it at a lower voltage your current drain will also be lower. You will probably find that you'll do all the maths (or math for you lot on the other side of the pond ) and it won't work out exactly how you thought. I would expect the fan's resistance to be reasonably constant and independant of operating voltage so if you run the fan at half voltage then the current drain will be around half of that quoted (or thereabouts). However, you might want to skip the maths and just experiment.

eg.
Fan = 12v, 50mA
R(Ohms)=V/I
= 12/0.05 Ohms
=240 Ohms

Say you want to be able to vary the voltage between 5V and 12V.
Current drain @5V = 5/12*0.05 Amps (assuming constant fan resistance)
= 0.021A (21mA)

The voltage drop across the resistor at minimum speed will be 7V (12V-5V).
R=V/I
=7/0.021 Ohms
=333.3 Ohms

W=I*V
=7*0.021 Watts
=0.147 Watts

Now then, near max speed say you're supplying the fan with 11V (ie dropping 1V across the resistor):
Current drain of fan = 11/12*0.05
= 0.046A (46mA)

Required resistance = V/I Ohms
=1/0.046 Ohms
=21.7 Ohms

Power dissipation = V*I
=1*0.046 Watts
=0.046 Watts

So the maximum resistance you'll need in this case is 333 Ohms and a max dissipation of 0.147 Watts.

Hope this is of some help picking a suitable pot.