As I read your post I observed two separate issues. First, you mentioned the journal and I didn’t understand you to say you required any word limit for that which I think is good. Journaling should focus in my opinion on thoughts, feelings, what she is experiencing, etc. and she should feel free to write as little or as much as she finds needful.

With respect to the writing assignments however, I think a word count is useful. It promotes efficiency and self-discipline in the writer. It prevents someone from writing too little and not adequately addressing the assignment and also forces them to focus and write what is important rather than rambling. Most writing courses do employ a word or page count for those reasons.

As far as subjects to write about, I think you must first decide on what it is you wish to accomplish through giving writing assignments. Giving a writing assignment just for the sake of giving an assignment I think is not very useful. Do you want to expand her knowledge on a particular lifestyle related subject? Do you want her express to you how she feels about something in particular? Do you want her to reveal to you her nature, likes, needs, fears? Once you have an objective firmly in mind, I think it will be much easier to think of suitable topics.

As an example I might ask a submissive to write for me a favorite fantasy so that I can gain insight into what her preferences and needs are. As an alternative, I might provide her a fantasy setting and require her to complete the story which would give me similar feedback. If I wish her to expand her general knowledge of the lifestyle then I might give her specific assignments on topics to research and then report on. (e.g. Discuss Erotic Humiliation) I would try and make these type of assignments as open ended as possible because as you move towards more focused specifics, I can see how she would find that more stressful. You could also refer her to a BDSM glossary and have her choose a topic that she isn’t familiar with to research and write about.

Limits I think are always fertile fields for writing assignments. Have her write about one of her limits and why she feels that it is an activity she isn’t willing to explore. This kind of assignment not only helps you to learn about her but allows her to learn more about herself.