While 56' can be "normal" for a flatcar, they also had shorter cars. 40' flats were common too.
I've found the thing you have to look at in "traditional size" rolling stock is the width.
I had a MTH Railking gondola that I thought was fine until I built a scale gondola of the same type, ended up having to make new ends and floor once I saw how out of scale the MTH car was.
I have 2 Lionel flats that appear to be fine models of cars around 40' long.