When designing your content, you are designing for both readers and search engines, that does not mean you focus on one audience exclusively, rather you have to balance both considerations to achieve your SEO and reading usability goals.
Many web editors suggest short snappy content is best for readers, and that’s correct, but also really depends on your visitor’s goals.
For SEO, longer articles at 600-800 are better, so the question becomes, how can you design a longer article, but make that article seem shorter?
Really you need context; otherwise you just aim for 300 words. Check out Bruce Clay’s website, and iProspect or all the leading SEO company websites, you will see copy rich sites. We might critique them for things they can do on reader usability, but we cannot fault them on SEO given their rankings.
Think about both SEO and reader usability, also think about designing your content for your audience and client. If one client wants 4 articles a month, longer articles are probably necessary, if 5 a week, shorter articles will be okay because what you lose in length you gain in the volume factor for the search engines.
In addition, think about why the reader is coming to the site; see this link on my blog about an article from Jacob Nielson on long vs. short copy.