siebel, I wrote the "responsibilities" based on my resume but included more accomplishments and more details because as you mentioned, the resume is a very high level summary and cannot include everything. Here is a chance for you to elaborate what you have done in detail.
Although they didn't specify the word limit, I still tried to keep it short and clean. Just 2-3 bullet points under each key point. and I had about 7-8 points.