Ventilation of your home darkroom should be given some serious consideration. I have worked in non-vented darkrooms in the past and they get very stuffy after a half hour or so. The fumes from the "Fixer" and other chemicals can't be good for you, and may cause health problems later in your life. I will give you my opinions and ideas for ventilation and you can adapt them to your situation.
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Light Tight

Exhaust and intake vents must be light tight.

Location

Situate your exhaust fan above or near your "Fixer" tray.

Air Tight?

Remember, that if you have a light tight door to the room, then it might also be virtually air tight. Take frequent breaks from your work and open the door to provide good ventilation during the break.
If the room is air tight then your fan or air conditioner won't be able to push air into the room without straining - the room needs an inlet, so that the fan can draw air into the room. A light tight air inlet is easily made on an inside wall of your darkroom. You can draw air from the rest of your house and pull it into the darkroom and then extract it via your exhaust fan to the outside of the room.

Installation Considerations

If you decide to install an exhaust fan, there will be a few considerations. First it just depends on where in the home your darkroom is situated. If one of the walls of the darkroom is also an exterior, above ground wall of the house, then simply mount the fan above the processing area, venting horizontally through the wall. Again this must be light tight so plan carefully. If the room is more in the center of the house, or below ground, then you have fewer options. You could possibly mount the fan in the ceiling between the joists and then vent along between the joints to the outside wall of the house. You should install a proper hood on the outside of the house to deflect any rain water and a screened opening will prevent rodents, etc. from entering the home. If you or your spouse is handy, then you can probably do this job yourself, otherwise hire a qualified heating contractor.

The intake size will depend somewhat on the power of your fan. You need an intake large enough to allow the fan to run without straining too much. I will have to guess here, but I imagine a hole the width of your wall studs and about 2 feet (0.6 meters) high, should be adequate. Its also ok if the fan strains a bit (slows down when you close the darkroom door). For the intake vent I would cut a hole in the wall the width of the distance between the wall studs - the hole on the exterior of the darkroom could be either low on the wall or high up, just as long as the hole on the opposite side is the reverse (one side high, other side low). By situating the vents in this manner, you should not have to worry about light transmission through the opening. The intake opening could be covered with a decorative grill.

Air Filtration

The intake vent will require filtration. You can fabricate a three sided wall mounted wooden bracket around the intake vent in the darkroom and slide into this a piece of filter material. This can either be a small ready made furnace type filter or just cut a piece to fit your custom size opening.

Fan Capacity And Control

I would suggest a fairly powerful fan - something a great deal more powerful than your average bathroom or kitchen range fan, the best would be a ceiling or wall mountable fan that fits between the ceiling or wall joints of your room. Also, as you are making a new darkroom, I suggest installing a variable speed control for the fan, as this allows you to control the speed and more importantly the noise from the fan. With the variable speed control, you can set the fan to high speed for a few minutes to clear the obvious fumes (fixer especially) and then turn the speed down to a more tolerable, less annoying rumble, or even turn it off or open the door for a while when the air gets stale or stuffy. Go to Home Depot or another home ware supply store and look at what they have available - something about 10 inches in diameter with a matching variable speed rheostat that can be wall mounted in a switch box in one of your darkroom walls.

Formula

As for the specs on my fan, I can't remember the CFM capacity as it was installed many years ago. Try this link - they have a formula for calculating the capacity (I have no idea where they got their numbers, or how reliable they are).

         CFM Calculation Formula