Divide the attic into two right triangles (i.e.,measure from the peak to the rafters, then from the point right under the peak at the rafters to the outside edge of the attic), find the area of each, and then multiply by the length. So, if it was 8' from the rafters to the peak and 10' to the outside edge, it would be 8x10x.5 or 40 sq ft. Then say it was 20' long, take 40x20 or 800 cubic feet. Do this for each side. You want probably at least a few air turnovers an hour. See what the manufacturers recommend for air changes. My guess is 5-8, but that's a guess.
If you have eave vents, you'll get some natural convection...aiding it can help with a fan.