Some fuzz on the borderlines. Originally the 85's like the ProStaff were mids and the Prince Oversize (listed as 110 or 107 sq in) was, well, oversize. Everything else was mid plus. The Prince Graphite Mid as a 93" is probably the upper boundary for mids. The 95's were always mid plusses, but with racket head size now growing and 98 and 100 becoming popular, more people are starting to consider 95's as mids.
Howard Head's original patent for the Prince racket covered a wide range of head sizes, so no one else was able to make rackets much bigger than 85 sq in (Pro Staff) or smaller than 125 sq in or so (Weed) super-oversize. so there were pretty much just standard (wood rackets - 65 sq in or so) mids and Prince Oversize.