A standard river barge around here is around 200'x40', @ 1/45 scale thats 53"x 10 "
Of course they typically raft these barges together into 2x6 unit rafts.
Mississippi river tows get much bigger.

A sea going barge at 438'x 105' @ 1/45 scale is 116.8"x 28"

Have a look around Towboat Joe's web site for more river barge sizes, and the size of the tows they put together on the Mississippi.