It's not a particulary big space, though much bigger than mine which is only 5x6 foot.
Basically a small extraction fan like those used in bathrooms would do the job.
Mine is simply in the hatch up into my loft. It less than ideal but it does shift air. As I use a Nova fibre processor I don't switch it on till the selenium comes out. What I can tell you is that in my tiny darkroom if I have the selenium tray out, and I've been in there for an hour or two without the fan on (sometimes I forget) I do get a bit of a head on me. If the fan is on, no such problem.
Also design and build your darkroom so it will not attract dust, and what dust does build up is easy to remove.