They can be any length. If you really have something to say, then say it.
But people might arrive at your blog post while surfing ‘news’. They don’t expect to spent a lot of time reading any one page, and are certainly not in the mood to start reading a novel. Then you might have to give a summary and point to your expose’ elsewhere.
The second reason to avoid a monolithic blog is feedback. If what you are saying can be broken down into smaller pieces, then do it! People tend to give feedback after they finish reading a post. With a long blog, covering too many issues, they are not as apt comment on things earlier in the blog. And the replies section of your blog may become a confused jumble because the replies do not all pertain to the same issue.