Nobody today would buy a four/five seats car as small as the old Mini or Fiat 500. Especially because also the average height (and fatness...) increased in the past forty years. I'm old enough to have traveled in the original versions, and believe me, no way such small cars could have a market today, even if made safe enough. Remember also those cars had almost no trunk space, you would have issue to store the weekly supermarket shopping into one of those old ones. There's a reason they were quickly obsoleted by models like the Fiat 127, Ford Fiesta, Renault 5 and VW Golf which offered more space (albeit they were still quite small, compared to today models) and a more versatile interior (especially the fifth door and the re-configurable trunk).
Only Smart attempted to build a very small car along those lines, but it was almost unsuccessful.