Jeff,
I am not referring to any standard gauge car. But rather I'm referring to the size of the coupler head; the size and mating surfaces.
As I interpreted the initial inquiry it was regarding the size of the visible coupler. After all, the inquiry was for a modeling project.
Over the years the height and mass and shank size of "standard gauge" couplers changed. They will all couple together. I saw this one time at the California State Park "Railtown 1897" in Jamestown. In the yard a car from the very early 20th century was coupled to a car built in the 1960s -- there was quite a difference in the mass of the couplers.
Brian Norden