A longer passage consists of a few paragraphs and a shorter one is usually composed of one longer paragraph. The method that you should use while solving an RC question is more dependent on the question type and less on the length of a passage.
If a passage is a one-paragraph text, you usually need to read the first sentences of the passage until you understand the main idea (up to 2-3 sentences). Stop when you start reading specific information. Then you should scan the rest of the passage for structural words/phrases. Sentences beginning with such a word/phrase mark the beginning of a new subsection and explain its function.