In addition to what Steve: said, another reason it's important that the fan be properly sized is that if it ISN'T, it will 'attempt' to draw air from the living space, i.e., from ANY nook and cranny in the attic that leads to the 'main house'. If this occurs, the fan will suck cool/warm air OUT of the living space and send it out into 'space'. Hence, the fan is a WASTE of energy.
What I mean by properly sized fan is as Steve: said too. It should work in conjunction w/soffit / eave vents and a ridge vent. If you have gable vents, they figure into the picture too. In short, it's better to have a fan that's slightly UNDERSIZED than one that's oversized. Measure the height of your attic as well as the floor size. Take those numbers to the Store and go from there. (The area of the attic is: 1/2 Base x Height. If you use the wrong 'measurement' for the Base, you'll get the wrong fan unless your attic has a square floor. So to be safe, bring all 3 dimensions w/you.) AGAIN, do mention if you have vents and what kind. You don't put a fan in an attic where there isn't any air to 'feed' it. (Remember, in this situation, the fan will try to pull air from the house!)
Good luck! If you have more ?'s, post back. My best to ya and hope this helps.