Michael, A fan's rating is generally for conditions of no restriction at the inlet or outlet. In other words if the fan is rated at 400 CFM it will not produce that flow with 6 feet of duct on the inlet and outlet due to the pressure loss. My darkroom is about 8 x 8 feet. I have two inexpensive bathroom fans mounted on the wall above the sink. They both feed into a 4" round duct (even though the fan body is 3") and then run about 10 feet to the outside with 2 90 degree bends. Ventillation is adequate but I wouldn't want any less.

I decided against mounting the fans in the wall between studs or in the ceiling joists. The reason was that I wanted to have access to them in the event a motor burns out. If they are in the wall or ceiling it may be difficult to get to them without destroying the light proof integrity. I also thought it would be easier to keep the fans inside the darkroom and then only deal with lightproofing the exit of my duct as opposed to lightproofing around each fan cut out.

Pay some attention to fresh air intakes. I have two 8x16 inch louvers through the wall. They have filters to prevent dust and a baffle arrangement for lightproofing. This may be too small since I can tell that the air flow increases when the door is opened which indicates that the fresh air intakes are restricting the flow.