subwoofer port

Hey, I have a Pioneer 12" subwoofer, and I am not sure how big I need the air port on the back of it. The enclosure is LxWxH- (in inches) 15.25x10.625x15.75. The volume of the enclosure is 1.035ft^3. Please help, Right now I have a port of 3", and its seems to want a bigger hole. The sub is 800...