Well, the "Baby Madisons" are 12.5" long (13.5 including the couplers) about 3.25" tall and 2.25" wide. They are a close match in height and width to the traditional Madison cars.
The 2400-series style streamlined cars are 11" long (12.25" over the couplers) and just over 3" tall and just over 2" wide.
These are Lionel cars. I don't have any of the other mfgr's O27 products.