I think this comment should be of use. The Tabula Peuteringeriana, which is almost certainly the map you're thinking about, stands awkwardly as both our only complete world map of genuine antiquity (though it exists in a copy) and yet is an itinerary map rather than a true representational map. That the Romans were capable of truly "to scale" maps is quite clear from looking at the distance tables of the geographers, and we have many surviving maps from the Middle Ages drawn from no source other than those tables.