Generally... they ask you about various features and design decisions that you made during the course of your work on Part II. Anything that factors into that design is fair game, from functional requirements to choices of technology, to ways to handle the non-functional requirements.
As for length of the answers, the general rule to follow is "long enough to answer the question without being unnecessarily verbose".

There's no cut-and-dry measure for what constitutes a sufficient answer to the questions in the essays - for the original
SCEA, I think most of my answers were along the lines of two paragraphs. For the new one, my answers were longer, but I also felt more confident about my design on the SCEA5 project than I had on the original.