There are not many people who can look at a ship model (plane, train, or even boat) and know whether the "people" on it are the right size. One of the "rules of thumb" I learned many years ago was "err on the small side". If you don't know the exact dimension of something, go just a bit smaller than what looks proportional. Compared to a modern freight diesel, how big is the USS Constitution? Try to visualize the two together. An error of less than a mm will go mostly unnoticed.