Need to replace rear drum

You can try looking at the door sticker or possibly a sticker in the glove box, although I think that applies more to trucks. Sometimes the glove box sticker will list optional equipment installed on that vehicle. If there were two sizes of wheels available, typically 14" or 15", and you have the bigger choice, it's likely you have the bigger brakes.

The safest way to prevent ordering the wrong parts is obviously to pull a drum off and measure it, but you should be able to get close enough by just crawling under the car and measuring the backing plate. With a 1 1/2" difference, it should be rather easy to figure out which size you have.