The larger the diameter of a fan, the lower the RPM it can spin at to move the same amount of air (same cooling capacity) as a small fan. Provided you can put the air where it's needed (e.g. a 1 foot fan can't "focus" air onto a 6 inch radiator) a larger fan will just about always be quieter and more efficient.

Oh, and having a water cooled PC with the radiator and fan inside the PC itself is silly. If you run the pipes outside or into your basement, your PC is almost completely silent, plus the cooling capacity will usually be much, much higher, because not having to cram fans and a radiator into a small enclosure lets you make them bigger and more efficient.

In my case I have a DDC style pump, and its very quiet after I got a car wash sponge and cut a square hole in it and put the pump inside. It looks ghetto but its inside a case so who cares.

At first I was worried about heat, but its lasted 10 years so far.