I understand leaving it to the author. I can agree with that, since if I ever made one I require a decent amount. Still, I think there should be zero circumstances when someone should post less than two sentence. Even if you describe the setting or scene in more detail for the sake of development would be fine. Also, normally one someone posts very short replies, a lazy chain reaction often happens. Then the quality of the rp plummets, to the point it may cause the entire rp to end. Maybe I just been rping too long, but I believe this is especially true in forum posts, where you have to wait sometimes hours for a reply, it isn't fun to see a two sentence reply after a long wait. I think anyone can post a fleshed out paragraph (4-7 sentences) at least. I guess this goes with the other members leaving off in a bad spot.
But yeah... If people don't mind short replies, I guess just leave it to the GM-OP, whatever you want to call it.