As big as you can get!
Acutally, you would probably need at least a 156W but a 172W or higher would be better. The highest wattage pelt that I have seen available from water cooling vendors was a 220W.
Also if you are going to put even a 156W pelt in your machine, you need to have it powered by its own power supply. Even a 156W pelt @ 12V will draw close to 14-15amps by itself. Most smaller computer powersupplies cant even handle that load without anything else attached to it.
Finally, those pelts will put out A LOT of heat (wonder why?
) so you will need a good water cooling system to handle the load.