I have been asking alot of people about this subject as Im the type whom hates buying the wrong thing.
After some email exchanges, I have been informed that the rule of thumb for choosing a radiator is 1 x 1 x 1" per horsepower.
Now this measurement does not take into consideration extra cooling ability needed for an AC condensor, nor internal oil cooling. This would be for the Engine only.
A Fiat X19 radiator might be sufficient to cool an EJ22, or even an EJ25 being non turbo'ed.
But if your talking about larger, turbo'ed. or typically hotter running engines, the radiator must be sized accordingly.
But another big thing to think about as well is how effectively your radiator is getting air. Its placement and efficiencey is paramount. Even a 10 foot radiator placed in a closed box wont work.
To give an example. The WRX STI Turbo runs stock at about 300HP, yet its radiator is worth 768 HP according to this rule. So it cooling needs to be over twice what is needed, to compensate for the AC condensor being placed in front of the radiator, and its built in oil cooler in the radiator, on top of it cooling efficiency given its placement in the car.
Most exotic cars run a ram fed style system, but with a good sized exhaust for the hot air. So its fairly simple to say how the air can get into the radiator, but another thing to consider how it gets out.
I have found the scirrocco styled racing aluminum raditors a better choice for my WRX engine.
they are 24 x 12.5 x 3 inches and are aluminum which dissapates heat more efficiently than steel.
Something to think about.