Figure out the size of your attic in square feet. You may have this information already on the floor plans for your house. If not, measure the width and the length of your attic floor, then multiply the two numbers.
Take the number of square feet in your attic and multiply it by 0.7. For example, if your attic is 500 square feet, you would multiply that by 0.7 and get 350. The number you get, 350 in the example, is the CFM (cubic feet per minute) minimum fan rating that you'll need in an exhaust fan.
Multiply your CFM minimum fan rating by 1.2 if you have a steep roof. If you have a dark roof, multiply it by 1.15. That will be your final CFM minimum fan ratings to look for when buying your fan, because these types of attics will need more energy from their exhaust fans to stay cool.
Buy an exhaust fan that is too big, rather than too small, if your options are limited. Most exhaust fans have between an 800 and a 1,600 CFM rating. If you need one with an 850 rating, but all that is available is an 800 rated fan and a 900 rated fan, buy the one with the 900 rating.