While this has been suspected for some time, it appears that it is definitely the case now. All things equal, a longer article will outrank a shorter one. Note that a longer article is several thousand words long as opposed to the average 250-word article. Many professional SEO marketers have identified roughly three areas that Google looks at for content:
This is the level of engagement and satisfaction that a searcher gets from a result. This is most likely measured in the bounce back to the search results. The longer the visitor stays on the page, the more likely Google believes that they are satisfied with the content. Longer stays also equal higher levels of engagement. Remember that Google knows by cookie and IP when a visitor goes back to a search and if the search is related.
Diversity of content
This is in comparison to all the other content in the search results for that query. The more diverse the content compared to competing articles the better this will be. The uniqueness of the content will also play a part.
There are probably many ways Google could gauge the quality of the content. Some of it could be a combination of engagement, diversity and the quality of the written English. It is not unrealistic to assume that there is a system within Google that can grade the quality of writing and style.