They used different sizes for different things. My best advice would be to use the Archer rivet decals for Pullman cars- the research for those was done by Tom Madden and Larry King- who quite probably measured several real cars. I am sure there are some drawings in one or another collection that would have the info, but could not find it in a quick look at the books I have handy.
When I was a kid, this sort of question was easy- all you would need to do was take a couple quick measurements of the rivets on the car you were riding to the suburbs.