I've done my reading, but there seems to be more than one answer for what size rheostat will work best. This is the fan I'm going to control. A 100ohm 5volt rheostat was recommended by the first site ...

As described above, potentiometers and rheostats are basically two approaches to the same problem. For both, the end result is the same, i.e. reducing voltage through a circuit. For a 120mm fan, you ...