Foremost thing is to answer exactly what is asked in your own words regardless of length and number of sentences.

What I do is to write a comprehensive, detailed, answer, so that it may cover all dimensions of the question. Resultantly, it becomes a short paragraph, covering almost 2/3 of a single side of a page. Moreover, as per my mentor's advice, I give heading to each answer in comprehension section. This also makes my answers look longer.