Passenger equipment, even on interurbans, could be as short as the 35-40 foot range--and trolleys, of course, could get very short indeed (gotta love those single-truck Birneys!) while box motors, while shorter, were strictly for freight.
I think there is some precedent for short gas-powered doodlebugs in the 40-50 foot range. And of course there are those Athearn Hustler body shells for a four-wheel single-truck doodlebug!