Selecting best Heat Sink for TEG (Thermoelectric generator)
The thermal resistance of the module is 3.33&deg;C/W, which means that at a &Delta;T of 50&deg;C, there will be about 50/3.33 = 15 W of heat flowing through the device.

What you need to decide is how much of your &Delta;T you're willing to "waste" across your heatsinks. In other words, you have a thermal circuit that consists of three resistances in series:

heat source
     |
     | thermal resistance of "hot side" heatsink
     |
-----------
     |
     | thermal resistance of TEG
     |
-----------
     |
     | thermal resistance of "cold side" heatsink
     |
ambient temperature


Your overall &Delta;T will be distributed across these thermal resistances in proportion to their values, just like voltage is distributed across a series of electrical resistors in proportion to their values.

If you want, say, 90% of your total &Delta;T to appear across the TEG, you need to select heatsinks that each have a thermal resistance that's less than 5% of the value of the TEG itself, or 0.1666&deg;C/W, and they need to be large enough to handle 15W of heat flow.

Note that fans on one or both heatsinks can considerably reduce their overall thermal resistance, but the power consumed by the fans will cut into your overall electrical efficiency, so you'll have to take that into account. You could use a Stirling cycle motor to power your fans directly from the heat, rather than using the electricity.