Jakob Nielsen has an interesting observation about the value of long articles (or posts) vs. short ones in the competition for reader’s attention. He finds that online readers prefer short posts except when they really need the info. So his final conclusion is that a mix of 2/3 short (600 words or less) and 1/3 highly informational long pieces is about right.
Now the stats this affects are both time on site and pageviews. If you want to increase pageviews, thus increasing ad impressions, considering writing posts in which the first 600 words stand alone as an article then use the ‘more…’ option to continue that article in more depth. You’ll straddle both worlds and keep all your visitors happy.