There is room for both, depending on the content and what you want to say.
Short posts – around three paragraphs – are great for when you have a simple observation or point to make, or you’re short of time (see the three things to tell a reader in How is blogging different?)
Long posts – for more indepth analysis – are best broken into manageable chunks that the reader can scan easily (see my previous post on structure).
A series is good for breaking up a complex subject, and can later be pulled together in a round-up post, eg, the Blogger’s Style Guide!
[FYI: Last year I wrote a Blogger’s Style Guide to help people in the organisations I was working for start writing posts and publishing them on the company blog. Many had never written anything beyond an email before but they did know their subject far better than I, so they just needed a good briefing in style, tone, structure and so on. This is that starter kit for company bloggers, consisting of 10 mini-posts in all.]