How about simply indicating how many characters the current field for the prompt will allow? I know that when I looked at it during sign-ups, I felt like I could only write five or six words for a prompt. If I know that it's X number of characters then I can compose my prompt in another document and be sure that it says what I want it to say. I think that would give me, as a writer, more flexibility to know I'm writing what the person wants *and* as a reader, I would feel more comfortable being able to give a sufficient prompt that conveys what kind of story I'm hoping for.