dear length of the answer doesn't matter, what u are writing in the answer it should be relevant what u have been asked and it depends upon the nature of question, if u can satisfy the examiner in 200 hundred words it would be enough, however considering the 20 marks for a question length should not be too short, normally word count 200-250 are enough, but keep in mind you have satisfy the examiner, don't write any irrelevant material to increase the length of your answer