Size alone is not the important factor. There are so many things that go into a successful cooling fan conversion.
In fact, I had a V8 in my S-10 using the stock 4.3 V6 radiator, and I mounted a pair of 12" pusher fans on the grille side and was able to achieve perfect cooling capabilities in the dead of summer with the a/c blasting.
The trick is, the fan has to be very close to the radiator. At the same time, there has to be enough flow through space all around so that at highway speed you can get natural air to push through. If the single or dual fans cover too much radiator space, you might overheat at 65 mph.
However, the tighter and better fitting a shroud is around the fan the better. So, you will notice many factory cars have little doors or flaps built into the shroud. These open towards the motor. When the cooling fan is running and frontal pressure is low the little flap doors stay closed. When the pressure in front of the radiator at high way speeds increases beyond what the fan can recirculate the doors pop open allowing air to free flow in around and out of the radiator fins.
So, if you can put together a single fan (16" is good) and a good tight shroud that hugs the fan and covers the back side of the radiator. Then put two large flaps/doors and or four smaller ones to vent off the high speed air, you should be able to make that radiator you have now work like a champ.
I plan on doing what you see noted above on the 69 big block. I feel one fan done right is the best way to go. I will build my shroud out of fiberglass.
for 20 minutes. I use a 180 thermostat and the fan is set to turn on at 185 and shut off at 170. The car heated up to around 185, fan kicked on and car cooled back down to around 170 at idle. Can't beat that. The only think I have not tested is in the dead of summer or with the a/c on. My a/c does not work and probably won't for many years. Many folks go with dual electric fans;however, for a mild 350 I think a high power single 16" fan works great and allows for better air flow at highway speed. Based on my dyno, I picked up 20 hp and 12ft-lbs of torque. I also think my gas mileage went up. 
