Hi
@MN,
You are very welcome! That is a great question. This was a concern of mine when I faced written exercises.
Unfortunately, this will depend entirely on the scenario/question and on the amount of time that you are given. My experience was restricted to written exercises in the form of emails to a supervisor. I tried hard to keep my answer to a single page, and if not, to one and a half pages. This can be difficult as often you have to complete written exercises using software provided by the firm. Admittedly, I did write more on one of my vacation schemes because I felt that a longer answer better suited the exercise.
My advice is think about who you are writing to and what you are trying to achieve with your piece of work in the scenario. What would your audience respond best to and find most useful? I.e., if it is an email to a supervisor, do not make it very long, as they are likely to have limited time. If it were a (detailed) research note, this might be different.
I hope that helps. I do appreciate that this is fairly abstract advice, so may be of limited value. Do ask if you would like anything further clarified