However, if your main aim is for it to get search engine traffic then the length is a factor. I have found that longer blog posts -- that is, over 500 words -- tend to rank higher in the SERPs.
Obviously, they can take a while to write. So this approach can make you feel like you're not really progressing. That's why, when you're just starting a blog, it's a good idea to write lots of short posts. When your blog is young and without any backlinks you've got little chance of getting any search traffic anyway. So you should just forget about trying to draw visitors this way for the time being, and focus on cranking out many information-rich entries.
You'll get a sense of achievement out of doing this, which is valuable. You'll also cover a lot of ground subject-wise. This can give you a good idea of the overall direction you should take. And the cumulative effect will help with SEO in the long run. Google will get to know what your blog is about a bit quicker than if you hadn't done this. It might help increase the speed at which it indexes the content, too.
This approach is a bit like a sculptor throwing down bits of clay to get the basic shape established. After that, he'll start being more careful and detailed in his approach and the true and complete character of the project will gradually take shape.