This is a great post, I know what you mean there is a fine line between how long a post should be and being too short.
I also struggle with this, but in my opinon what you have to think about is the content you are writing about. As much as we would love to generalise everything we simply can't so thinking of some examples here.

Let's say you wanted to write about cancer, its a very broad subjects. There are so many different types of cancer, then you have the symptoms and the treatments available. So you can either write one huge post which some people will read, because its a subject they feel strongly about. Or you could bitesize it into many shorter posts.

Now lets say you wanted to do a post about teaching someone to make a sandwich, your post is going to be shorter and more punchy, you are probably going to put some pictures of bread and filling and shorter paragraphs.

So my advice is go with how you feel, does the content need more words, or are you wanting to make a specific point.

I hope this helps.