The 'Big Hauler' flats were all one size, but I assume you are also referring to the shortie 20' flats with the short trucks?
Here's my pair (one of each.) As Mark says, paint the deck (or replace it with real wood.) My flat has a pair of EBT end gates and a lumber load. EBT put any old stick in the stake pockets, so I did too.
I found this photo of the whole train, to give you another view.