The length of your content goes hand-in-hand with its quality, at least in the eyes of Google. While it’s been proven that the average surfer these days has a very short attention span, Google seems to thing that very, very long content is “quality”, and short content is not.
It used to be that putting up ten 150-word articles would bring in far more traffic than putting up one 1,500-word article, but those days are gone. These days, content should be long—the longer, the better. Not only that, but Google seems to favor content that is actually updated on occasion—meaning it’s better to add a bit of content to an older article than to create a new one—at least once your site already has a good amount of content on it.
Generally, articles should be at least 500 words, and up to 3,000 (perhaps longer in some cases) is even better. Who knows why Google seems to be so adamant about longer content considering declining attention spans, but as it is, length matters.