Each blog article should be 250 to 450 words.
Topics are up to you. Keep the topics relative to your audience. Topics can be news, information or anything your audience will want to read.
The blog is targeted for people on the site first and search engines are second. Nevertheless, keep search engines in mind if you want to appear on them. Writing for the Web is slightly different than academic writing. Words should be repeated throughout articles if the article still makes sense to the reader. So if your article is titled, “Dating Tips for Women”, for example, then the words “dating”, "women" and "dating tips" can (and often should) be used throughout the article you’re writing as long as it isn’t used too much so that it skews the overall message. Doing so will help people find your articles when they type “dating tips” or any combination of the words "women", "dating" and "tips" in Google.
For more information about SEO (Search Engine Optimization), click here and read this article.