the length of the answer matters little than quality of the answer.
but the length of the ans. must be long enough to compel the examiner give max. marks...e.g u cannot write just 2 pages for a 20 marks question. it would be inappropriate and will not appeal the examiner.
the examiners of css papers are selected ones,talented and i m sure they prefer quality instead of quantity.....

best of luck for all of u..