Undergraduate History ExamWatch
As for how much to write, how long is a piece of string? Write as much as you need to answer the question, and not a single word more
Assuming the marks for each essay is the same, you'll probably have an hour to plan and write each essay, so figure out how much you can write out in that time and make sure it is complete i.e. has a coherent argument with introduction and conclusion and so on, and doesn't just ramble on then stop because you ran out of time...