Just reinforce the interview answers and the skills and/or achievements that relate most to the qualifications they appear to be interested in the most. I use a summary on my resume which is just four or five bulleted sentences that outline experience and education that fits the position. As it is, a resume is a one-page summary/outline of your experience and education, so I would assume that a further summary should only be a small paragragh. A resume is typically just used as a tool to get an interview, so it's kind of dumb that they want another summary, but maybe it's to pass along to the next person that may interview you, that may be more focused or have les time to go through your paperwork.
