I m looking for a best radiator fan for my new Koyo half size radiator which just designed for drag use only. I don't know what size to go on it and the blades design.

 

AFAIK, most aftermarket slim 12" fans will work. So will the stock fan.

Something with a shroud will work much better than unshrouded:

http://www.streettunedmotorsports.com/parts/fal_fan.htm

 

For drag only id say a 10 or 12, would be fine. Blade design you definetly want straight as they will flow more than curved. Don't forget that depending on your turbo setup you may need to get a pusher fan versus a pull type.

 

SPAL 12inch puller, that fan moves serious air for its size. I have one on full aluminum size rad my idle temps never go more than 195 degrees. 70% water 30% coolant..