There's a little chart in the upper right corner during edit mode. HP likes you to hit several checkboxes of content, not just lay down a bunch of paragraphs. So, use that thing as your guide. I usually find that I do about 900 words per Hub.
One kinda dumb thing is that if you make a recipe hub, they enforce a minimal word limit. So, I find myself adding unnecessary information about the recipe, just to fill the page.