There are two ends of the spectrum:
Super-short, like Claudia Harrington's blog
of inspirational short quotes from professionals in the world of Children's literature are like inspirational tapas - just little tastes, and don't take a lot of time to either create or read.
Super-in-depth and thorough posts that are almost like great magazine articles. A number of bloggers take this approach, and Colleen Mondor at her Chasing Ray blog does a lot of this -
a great example is her blog post series called "What A Girl Wants" which asks the same question of a number of female authors - and is fascinating reading. These posts take more time to read, have more substance - and often inspire more comments.
Certainly not every post on your blog needs to be the same length - and in fact, variety is kind of nice. But whether it's short or long - make sure you're offering something of value to your readers. Inspiration. Thought-provoking analysis. Resources. Something you care about and offer to share that no one else does in the same way. That's what makes for good reading - of a blog or anything else for that matter!
Remember to have fun, and keep it Zen.