How Much Do I Write?
When I was in college my public speaking professor had a cheesy saying for his essay requirements. “How long are your legs? Long enough to reach the ground.” Having survived arduous AP classes in high school only a few months prior, I was more than pleased to to turn in a one paragraph “essay”.
You blog posts only need to “reach the ground.” Whether it takes you 5,000 words or 20 words to get there is entirely up to you.
You should keep in mind, however, that reading your brilliance on a digital display is taxing on one's visual organs (and the Kindle to Blog audience is rather small), and your readers' attention spans are limited. It's nice to sit down and savor a blog post, but it takes time to cultivate an audience, and you won't be able to do that with Melville length screeds.
I think a good guideline is no less than 200 words—unless you have a really good reason—and no more than one page scroll. The Internet has its own “below the fold” and you'll want to make sure everything good is above “the fold.” The fold ends where you start to scroll to read more. This length is subjective, since font and display sizes vary, so make your best judgement based on what you see in front of you.
You'll find a good middleground as you go.
How Long Should A Blog Post Be?, Pro Blogger
3 Things to Consider When Deciding How Long Your Blog Posts Should Be, Copyblogger