It depends on how much marks the paragraph carries. If it is of 5 marks then 200 words will be enough but if it is of 7 or 8 marks then we have to write around 400-450 words and if it is of 10 marks then we have to write 600 words. Hope you got it.  
Write 4 to 6 sentences per paragraph :in that number of sentence you can announce an idea ,prove that with evidence and  explain why this evidence matters by linking it to the overall goal of your paper  
it depends on the matter and the points of the question you have to write at least
5 sentence per paragraph and 80 to 150 words per para