The longer side is usually the one running length ways along your vehicle, so make sure you measure to see which is going to sit the best. These things are great, and incredibly cheap. In fact, you can get a good quality one for under $300, and fit it in around an hour.
4WD Awnings are designed to be set up by one person, and very quickly. The procedure normally involves unzipping the case, un-clipping the supports, rolling the awning out and standing the legs up. The legs are incorporated inside the aluminium frame that the awning rolls out of. Stand them up, attach the supports and if it is windy, put a few pegs in the ground with guy ropes going to the awning. I usually suggest that pegging it down is a good idea, as a gust of wind will pick the whole thing up, and dump it on your vehicle. This often results in panel damage!
4WD Awnings are completely weather proof; I have had a puddle of water build up on our awning when it was raining severely, and not a drop came through. Whether its a quick bite of lunch that you are after, or you want to sleep under the awning, they are a brilliant idea. You can get Awnings that pull out from the rear of the vehicle, and awnings that will cover all 3 sides of the vehicle. Expect to pay a considerably higher amount for these, but if they suit your purposes then that is great!