I have a small attic above my 2 1/2 car garage. It gets very hot up there sometimes and I wanted to add either a small solar fan, or a whirlybird turbine ventilator (shown below) but I don't know the math formula to figure out what size fan to use. Because the roof comes to a point, how do I future in the height?

Also, how well do these whirlybird turbine ventilators work? I've seen them before but can't imagine they remove much heat. Can anyone explain how they work? Thanks.