I don't put in a lot of effort while writing posts, but sometimes I just start typing and next thing I know I end up with quite a long post.
The length of the post generally depends on the topic, if it's something I'm passionate about, interested in, or if I begin to type up my life stories, I tend to make long posts. Sometimes, I write a long post, just to try and explain what I'm getting at, then later I realize I could say it in just a simple sentence. I think I prefer long posts, really.
It's funny because I'm not god with the whole talking thing, but on this forum, I'm not good with the whole short post thing, I just start 'talking' and I keep going.