You will want the size fan that will best fit the size of the fin area on your radiator...but in addition to looking at the fan diameter, it's equally important to make sure it has a high CFM rating. I've seen some of these large fans that don't flow much more then about 1500 CFM--IMO for proper cooling with an electric fan you should have at least 2000 CFM. The core and fin size of the radiator is important to cooling also, as well as if it's a single, double, or triple pass radiator. With all the different companies selling aluminum radiators and electric cooling fans, there should be a lot of specs that go into selecting the proper components other then just the price......Also, as Roger mentioned, shrouding the radiator is important---if all the air coming in the front of the car passes over, under, or around the radiator it's not going to be doing much cooling of the radiator!!! A final thought, there also needs to be enough open area to allow the hot air surrounding the engine to exit the engine compartment.....