Make your articles as long as necessary.
The problem with telling you a one-size-fits-all ideal post length is that it all depends on what you're writing, your niche, your target audience, and how you plan on writing your article.
Writing an article that's too short can cause a user to find it uninformative and bounce off the page, or it can encourage a user to browse other parts of your site because they like your "straight-to-the-point" style. On the other hand, if you make the post too long, you may bore users off your site and back into the SERPs (where they'll look for an article that gets to the point), or the readers will really like the content and continue reading your other posts.
That's quite the conundrum, isn't it?