I've found that a 2k pot generally suffices for voltages in the range of 0 to 12V. You will never get full input voltage at the output because of the internal workings of the three terminal regulator, so you will never get the fan up to full speed, should you need to. At that point you want to jumper around the regulator circuit with a switch. Finding a 2k pot is more difficult than a 5k pot, but you can change the 5k pot to 2.5k by simply soldering a fixed 5k resistor across the end lugs. You definitely want to use a linear taper pot as opposed to an audio taper. Here's a rough graphic for changing a 5k pot to 2.5k
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