You are in the ballpark. Heavyweight passenger cars ranged from 40' to about 81' over the buffers - confusing, sometimes, because dimensions are often given of the interiors.
Harriman cars included a 40' RPO, 60' coaches, 72' diners - you get the idea. Usually "all baggage" cars were shorter than coaches or Pullmans.
I assume you are only asking about steel cars - you picture steel sided models.
Bob, Among many things I don't understand is where the 18 inches MTH/Atlas refer to is measured. And why are scale cars listed as Madison cars? I thought scale cars were heavyweights and Madison were toys. John