In "The new darkroom handbook" it is mentioned that the air the darkroom should be changed every six to eight minutes. Using simple math you can calculate the volume of the room and find out which fan you need.
What is (I think) important, is to place the exhaust (suction) fan on the wall BEHIND and not the ceiling above the sink (chem. trays), because this way the fumes will not go upwards in their way out (upwards is the way our head usually is, because we bend over the trays).
I made the mistake to put it over the sink and have regretted it...