Hi Chris,
our research has found that short posts AND excessively long posts generally don't rank well in search engines. My best advice is to make a blog post no shorter than 250 words and no longer than 1000 words. (Of course, I haven't broken into Google's offices and stolen the secrets to their algorithm, so I can't know for sure, but our research is a pretty good indicator of how Google thinks.)
Apart from SEO, there are several other criteria that determine the length of your blog post:
Reader attention span - it's pretty short on the web, so the closer you can keep your post to 250 words, the better
Quantity of blog posts: the more you post, the shorter your posts should be. If you update your blog with several lengthy posts a day, chances are that your readers will simply tune out some posts.
Comprehensiveness: if you write about a complicated topic, make it long enough to cover the topic in depth. Otherwise there is no value to the user. If that means that your blog post will exceed 1000 words, break up your post in several shorter posts that link to each other.
And that's my two cents...