Hi,
Several model manufacturers, notably the Athearn "Heavyweights" were selectively compressed in length to satisfy the common 18" radius of many model railroads.
Actual passenger car lengths gradually grew, as the advancement of better carbuilding materials and technology developed, from a common length of about 50 feet in the 1870s during the era of all wood construction. Around 1905 steel underframes became the industry standard allowing carbuilders to approach seventy feet in the length for their cars.
In 1907 the PRR, out of necessity due to a New York City ban on using wood cars in the North (Hudson) and East River Tunnels built their first P70 coach at an outside length of 80' 3-3/4"
By 1915 the standard heavyweight design allowed a length of 85' give-or-take and for practical reasons this length became the norm.
ACF lot #7789 003 by John W. Barriger III National Railroad Library, on Flickr
Some head-end cars were shorter with some being in the 60 to 70 foot range. By this time the US Post Office was requiring RPO cars to be of all steel construction.
If you can find a copy of The American Railroad Passenger Car by John H White, Jr. (two volumes) you will find a great deal of information on the development of North American railroad passenger cars.
Good Luck, Ed