I was urged to write a lukeprog-style Main posting on parenting (I may) so I take that to mean that postings with more than 6000 words are OK—if they have a high quality.
And you probably shouldn’t try a Main posting if it has less then 1400 words.
Reading only the introduction of “A Crash Course in the Neuroscience of Human Motivation” (which incidentially is by lukeprog) reveals that it’s a very special case and additionally an experiment of a long post.
At 10,000 words (not including the notes), A Crash Course in the Neuroscience of Human Motivtion is one of the most highly upvoted articles at the site. But so is Your Inner Google at only 300 words.
Reading only the introduction of “A Crash Course in the Neuroscience of Human Motivation” (which incidentially is by lukeprog) reveals that it’s a very special case and additionally an experiment of a long post.
And while “Your Inner Google” is interesting and insightful it is criticised for being too short in a much upvoted comment: http://lesswrong.com/lw/7mx/your_inner_google/4u6t
This doesn’t say that you can’t do such posts—but you should really consider the reasons for doing so.