Heatsink capacities are 150 and 750 for classic heatsinks (meaning that the larger one is more space-efficient).

Anti-entropic heatsink has capacity of 200 (so it is even more efficient). Personally, I haven't noticed any energy production either, but it is overpowered anyway - since with these you don't need to build radiators and worry about rooms overheating anymore.

One more tip: Emergency purge ports have heat capacity just like heatsinks. They will still overheat your ship. Keep them outside of the ship interior and you won't have the problem anymore. They will just stay stuck at their max. heat capacity (which is very small anyway) and still nicely purge heat from your anti-entropic heatsinks if necessary.