It measures 2 1/8" by a 3 1/2" more or less. Here it is with a standard card for perspective.
I found this page with a detailed look at the set. Some of the 'Mrs.' photos are a hoot. Well worth a peek. If you stumble upon a Jim Grabowski or Mrs. Joe Morrison game piece Amoco owes you a 1969 Ford. Ask for a Mustang.