This is interesting...I usually know how long a story will be. I think it comes from my stint of newspaper writing when the editor would say, "I need 500 words of filler, now." And then he would watch over my shoulder as I wrote it. Usually stories appear in my head with beginning and end. My job is to find the middle bit -- the journey getting there.
The wonderful thing about a round robin is the surprise of not knowing where we are going!