It seems to me that most contributors to this thread assume at least implicitly that vocabulary size stands for a certain quality of a person: the larger the better.. This is certainly valid up to a point. Rather than speculating about the maximum possible vocabulary size, it would be interesting to know the "most useful" size, i.e. the point where further increase only appeals to collectors of rarities, and does not contribute anymore to the precision of communication. Accessibility of information quite generally decreases with growing volume.