Heatsink selection and datasheet specifications for Thermoelectric Generator
That spec sheet is aimed at using it as a cooler, not a generator. I don't think you can infer its generator performance from that, but you might be able to find better datasheet for a similar model. Short answer is : try it, but don't expect much.
This is about the best datasheet I have found : scroll down to TEG2-126LDT. (Available from that site; follow the links for more data).

Th=40, Tc=30C, Voltage at match load 0.2V, Amps at match load: 0.045A, Match load 4.5Ω
Th=60, Tc=30C, Voltage at match load 0.6V, Amps at match load: 0.11A, Match load 5.2Ω

So, at dT = 10C, you can expect 9 mW, and at dT = 30C you can expect about 66 mW, both into loads about 5 ohms. At 20C you'll get somewhere between these values using their module. They chose it for this role; yours may offer a bit less.
As for the heatsink required : it is whatever you need to maintain that temperature difference. The full datasheet shows heat flow across the module as 52W, but that's with a dT of 170C. You can expect heat flow to scale nearly linearly with dT, so I would suggest designing the heatsink to allow for at least 10W at 20C or 5W at 10C.
Now you have a hot source at T2 and a cold source (presumably air, since you mention a heatsink) at T1, and a dT (total) of T2 - T1. Some of that will be dissipated across the heatsink - dT(sink) - leaving dT(TEG) to generate power.
dT(TEG) = dT(total) - dT(sink).
Then for example, if you have a 45C heat source and 25C air temperature (dT(Total) = 20C), and you need 9 mW so dT(TEG) = 10C, then dT(Sink) = 10C and dissipation is 5W. So you need a 10C/5W heatsink = 2C/Watt.
Or for 66 mW, you need dT(TEG) = 30C, choose a heatsink to dissipate 15W at 1C/Watt giving dT(Sink) = 15C. If your air temp is 25C you need a heat source at 25C + 15C (heatsink) + 30C (TEG) = 70C.