If writing for your site, write or hire someone to create a long, authority 'pillar' piece per each major topic or keyword. Add a few shorter pieces as 'satellite' articles to the main piece, and put in good internal linking to support the pillar. If just writing in general, go with the length suitable under the circumstances---if it's for a editorial level niche site, make sure it's long enough to satisfy the specs outlined by the domain, or if it's a generic platform, write shorter pieces, and so on.