The GRE screen is at a resolution of 640x480 pixels. So a passage that seems very long isn't really that long. Anything below 50 lines is short, above 100 is long. Time is precious and you don't want to be wasting too much time on RCs. For short passages, read first and then answer the questions. For long ones, read the questions and search for the answers - you have a much bigger chance of getting the answers wrong here but you save a lot on time. Again, questions based on the entire passage (like author's mood for example) are hard to answer this way, because you need to know the entire passage - but for a long passage such a question would usually be grade 5 - so you would hardly lose any points for getting it wrong.
