First, calculate the number of watts of heat to be dealt with, using the values on your fan labels with one of the Power formulae,
With linear fan control, the heat produced in the controller chip is at a maximum when the fan is running at 6volts, leaving 6v across the controller.
At this level, the fan is using about half its 12v-rated current, and around a quarter its rated power, so for a 0.25A (3W) fan the heat produced in the controller at 6v is 6 * 0.125 or 3W/4, both of which equate to 0.75W.
Next look at "worst-case" conditions. Taking a hot summer day's with the room at 40°C (104°F), to keep the chip temperature below 90°C, the maximum temperature rise allowed is 50°C.
Now we need the maker's data for the chip package thermal resistance between its junction and case, and a figure for the thermal resistance between the case and heatsink. The first is around 4°C/W for a TO-220 LM317T, the second about 0.3°C/W using a thermal compound.
The required heatsink will have a maximum Thermal Resistance (lower is better) of:
(Max Temperature Rise / Watts dissipated) - (junction-to-case resistance + case-to-heatsink resistance)
= (50/0.75) - (4.0 + 0.3) = 66.6 - 4.3 = 62.4°C/watt.
In this example, virtually any small heat-sink would be fine – in fact the 317T has a typical junction-ambient (ie, no heatsink) thermal resistance of 50°C/W so you'd get away without one, though the chip would be far too hot to touch. But add another fan, doubling the heat produced to 1.5W, and required heat-sink would need to be 29°C/W. Maplin's KU50 (25°C/W) or RN84 (20°C/W) would suit.
If you wanted to cover all possible future needs, at the 317T limit of 1.5A at 12v, or 0.75A at 6v, heat produced will be 4.5W and the heatsink needed a 6.8°C/W monster, something like Maplin's RN90. But that would allow 18W of 12v fans, though you might be better off splitting the fans between a few more controllers.
These are the PCMods rheobus sinks. The unit is rated at 17W of fans per channel, so using the above calculations the sinks need to be about 7.4°C/W.
Pictured right is Marston's 6°C/W model, similar style but a tad taller, so it looks like PCMods got their sums right!