I see this question asked time and again in the forums.
The simple answer is ideally 250 to 450 words.
Knowing that you have only 250 to 450 words in which to get your point across helps you focus your content. package your points in a clear succinct and effective form. This gets rid of all the rambling and makes each and every sentence valuable to your reader. Meaning... You get there attention
Search engine optimization
But sometimes you really cannot cover a topic in a blog post that is 450 words long. Especially if you want to write a pillar article that really exhaustively goes over all aspects of a topic.
So what do you do then?
We'll cover this in a future post. :)