What’s a good rule of thumb if there is one?

This is a subjective subject, but these are my thoughts

•  Remember you are writing for your readers not a search engine.
• If you can get your message across with a couple of lines and an image, it’s still a blog post.
• In some market sectors 70% of your website visitors are using smart phones, so shorter posts broken up with bullet points will be shared more and gain more traction.
• Wild card  – use video.  A two minute video will equal a side of A4 but will be easier to digest and share.
• If you are writing a blog post and it keeps getting longer you will find that you have covered  several points that could actually be shorter separate posts.