What size rear speakers come installed in a 1989 Chevy caprice?

The 1989 Chevy Caprice didn't come standard with rear speakers. Many Caprices were equipped with only a single front speaker system, or perhaps with optional front and rear speakers of varying sizes depending on the options package selected by the original buyer. There's no single standard size. To determine the size, you'll need to physically inspect the rear speaker locations in your specific vehicle.