I always try to keep my word count around 1000-1500. I find that after you get higher than that, people start to lose interest and don't read all of it, often missing important details. On top of that, once you start writing posts that get up into the thousands, are you really interacting with other people? Or are you just writing the story that you want? Shorter posts around a thousand I think promote interaction between players as well as keeping people from getting tired of reading.
(Sweeps nearly 3000 word post I just wrote under the rug)
Shorter posts around a thousand are better! :D