I am sure that for that processor the biggest one you can get, the better. However dont overlook the fact that with that CPU and with say a 226W peltier, your watercooling loop will have to be able to handle a heat load somewhere around 300W+! Keep in mind that when youare using a pelt, you are removing the heat from the CPU (between 70W -> 100W) AND the heat being produced by the pelt, which can be as high as the rating of the pelt itself (gurus out there, correct me if I am wrong).
Also, GET A STRONG SEPARATE PSU for the pelter. I know a few guys around here that use 15V PSU rated for 40A. Probably not a bad idea to have a bit of overkill in the Amperage range when setting up this project.
One last thing, make sure to use LOTS of insulation to keep condensation away from the MB.
I will leave the more indepth details to our pelt gurus out there
Good luck!