[@Shadow Daedalus]

You're expected to write as much as you need to make your fellow players' posts easy, and to push the story forwards. I'd estimate that you'd probably want a few hundred words for that, but if you're someone who writes in very concise, very elegant prose then who am I to judge? Make it good, and make it enough to properly serve the story, and I'll be happy. I usually end up writing ~500 words per post, but that's just me: everyone is different.

The only hard requirement is grammar and prose that are not painful or frustrating to read. Consistency of tense, proper spelling, no unintentional grammatical ambiguity and things like that are the only absolute requirements. I'd put more in, but honestly if you want to write like James Joyce and you think you can do it well, then that's fine by me.