I personally hate fans, I hate the noise. The larger a fan slower it has to move, the slower it moves the quieter it is. Unless you have some crappy fan that is loud because its built poorly. If you take 2 fans from the same maker, the larger one will spin slower and be quieter.
Long story short. If you are going to add a fan, get the biggest one you can find. It will work just as well or better than a smaller fan, and be more quiet