I don't think there's any standard that I've come across - some of my blog posts go on for ages, others are just two or three paragraphs (same for other people's blogs that I like to read).
The most important thing is that every word counts (ho ho). It's always worth proofreading each post several times and cutting out unnecessary words, so that you end up with a tightly written piece that flows well. That way the reader will keep reading, however long your post is.
A good tip is to read it aloud - you'll soon notice if you are stumbling over any awkward phrases. And if your sentences are so long that you have to pause half-way to take a breath, divide them up!
And of course, there are lots of really good posts around that are simply a photo, with or without a caption (or does that count as a picture speaking a thousand words?!)
Here's a link to my blog if you want to take a look: Young By Name
Looking back, the earliest posts were very short - and they got longer as I gained confidence and found my voice (pretentious, moi?)
Good luck - and have fun, in however many words it takes you!