Here's what I've done, so maybe you can judge fan size accordingly. HVAC consultation is a good idea. My darkroom, for b&w only, is about 7.5 x 14.5 with 9 foot ceiling, nearly 1000 cu.ft. I installed a 12x12 Doran fan (400 cfm) with its appropriate louver and installed the Doran speed control (rheostat) to exhaust the room air to outside. I placed it on the wall near the location where the fixer tray would reside. On the opposite wall is the inlet air that is filtered with 3M Filtrete filter, changed about once a year, incoming air is drawn from living area (laundry, storage cabinets, cat litter box, etc.). This arrangement creates a cross-flow and I've never noticed fixer fumes being bothersome, and I've never had to set the fan speed above about half power. I chose this negative pressure system because I was concerned that a positive pressure method would force objectionable fixer fumes to exhaust into the living area. The darkroom has stayed remarkably dust free. Before I installed the Doran fan, I found it to be very quiet just holding it in my hands, and I paid great attention to isolating its vibration when supporting it in the wall. However, the use of a standard outside aluminum vent cover (with flapper door) and tunnel through the wall creates a lot of noise from air rushing through. So I intend to replace the aluminum stuff with plastic materials and cushioning to reduce the noise. Since you're in MT, you should also think about keeping the room warm during winter sessions. I installed a thermostatically controlled electric wall heater (with small blower) because the fan will also be exhausting warm air. This heater only runs occasionally as a chill chaser; we don't usually have very cold winters here in the Seattle area. The glowing red coils are not visible in the darkness due to the louvres on its grill.