Normally a resistor would be a bad choice for this. I suggest you use one of two other ways:
1. Get a voltage regulator for 6V. I know there are 5V and changable regulators so I guess you can find 6V somehow.
2. Connect the fan between 5V and 12V giving it 7V. Close enough?
A decent voltage regulator with heatsinking will cost as much as a new fan with a built in speed adjuster. Any reasonably sized variable resistor will burn up under the current (0.5-0.7 Adc) a fan draws.
The resistor idea works fine. I've been using it on a noisy fan for the past 6 months. I put 2, 10 ohm 10 watt resistots in parallel and it drops the 12 Vdc down to ~9.5 Vdc which turns the 6800 rpm shrieker into a nice quiet 5200 rpm fan and it still does a good job cooling. Radio shlock has 2 pack power resistors for $1-2. You could get 5 ohm and use a single one but I used the 2 in parallel to spread the heat.
NOTE: A 10 watt rating on a resistor means that it can handle 10 watts when attached to an infinite heatsink with a 72 deg F temperature. I taped my fan slowing resistor to a metal channel running across the PC to help keep it cool.
Or just go buy one of the new fan control modules for $20 bucks or a volcano 9 with its own speed control for $20.
sure, it still moves air. you can feel it. a fan at 7v runs roughly 60% the speed of 12v. if you want something between 12v and 7v, then i think you're only choice is a regulator or resistor (not sure what the difference is).
Ep, glad to see you come back and tidy up...did want to ask a one day favor, I want to enhance my resume , was hoping you could make me administrator for a day, if so, take me right off since I won't be here to do anything, and don't know the slightest about the board, but it would be nice putting "served administrator osnn", if can do, THANKS