There's no real minimum post length, I just ask that you make sure your post goes somewhere and there's a point to it. Also try your best to include detail and description, painting a picture with your words. The most help you could get is just looking through other RP posts, though. I would highly recommend just reading through RPs if you have any sort of confusion on how to post. Of course you're going to have to read through the previous posts for this RP so you can be caught up on what's happened so far (which isn't much at all), but if you still don't really "get it", you can always just look through posts in other RPs.