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If I can use a 5w without burning it up I will, but I dont want to take a chance unless I have too. I need some help with this!!
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Divide the voltage of the fan by the current it uses (.65amps). That gives you resistance of your fan (18.5ohms). Then divide the max resistance of your rheostat by itself plus the reistance of the fan (rheo/rheo+fan). Finally times that by the total power use of the fan (7.9w). That gives your power dissapation of the rheostat. For safety's sake, try and be a little bit above that number. Or put a sink on it like I did
For example a 100ohm rheostat will need to be rated at least 7w, because it uses 6.66w at 100 ohms. A 5w 25ohm rheostat would be Ok in this case (it would use 4.5w), though it will get a bit warm.