Here are some actual tin-plate billboards . they measure 51/4" x 31/2" so they are probably o-scale .I have no idea who made them and there are no markings on them . I got them on the bay.
I also included a home- made billboard from the mid 1930's. made by my uncle who had a standard gauge lay out before the war .Half of my family worked at Exide Battery in Philadelphia Pa.