Having looked at the sample HSC paper for 2007, it would be either a 6 mark or a 20 mark.
I suspect it would more likely be a 20 mark, since it is "evaluate" and it asks about BOTH "contribution" and impact".
My opinion:
If it is 6 marks, write 1-2 pages (250-1000 words).
If if is 20 marks, write 2-4 pages (500-2000 words)
Mr. G. Mitchell
Pacific Hills Christian School
Studies of Religion Coordinator