A lot of what I write is referential i.e. I anticipate someone starting to read the article and either thinking "I'll refer to this as required" or "That's the bit of information I needed - and I'm done". Because of this, my posts can be quite long (sort of like a Wiki article); some of my longer ones are in the region of 7000 words.
That said, I publish news tidbits every now and then which are significantly shorter, in the region of the 500-800 mark that @FF88 mentioned. That's about as short as they get.
I wouldn't necessarily say I "aim" for a particular length. Eventually the article feels finished - and that's how long that post should be.
7000 words is insanity, might as well just rebrand it into an ebook or something! I tend to keep it at 500-800 words at max, but sometimes even shorter ones will work.