Ok fuck it I'll bite. We already covered the the M in MMO doesn't refer to instance size, so let's rephrase. How exactly does the size of an instance specifically impact on the quality of an MMO? What's the relevance? How do we determine the optimal instance size? What are the lower and upper bounds? If 8 is too small, why is 20 just right? Why not 23?
Putting aside the fact that you could have taken the spirit of what I said, which is ultimately that the specific size of an instance is nothing more than your preference and is not in fact a characteristic which inheres in MMO quality, what exactly is the specific impact on quality that an arbitrarily assigned number of players per instance is supposed to have?