oh boy.

does the contest specify the target resolutions?

If not, then what other uses do you have for nit? BEcause the question is general enough, so the answer will be general: Whatever you like to use it for, that's what dictates the ideal rendering resolution. If you plan to post it on youtube or kyte or similar places, and want to see the 'HD' button there too, then you had better render and upload an HD res. Like 720p (1280x720 progressive). If you want to play it in full HD on big screen, 1080p (1920x1080). If you are a gamer and want to push it to the full extent of your monitor's capability, 1920x1200, well that's the answer. And of course if you prefer to target users of devices like iPhone, iTouch, others in .3gp format around 200-400 pixels, pick a few of these.

I recommend rendering high so you can resize&resample down to lower reses too. The opposite doesn't always give you satisfactory results, to say the leats.

If you have just one video size you want to render to try 1280x720 if using wide format. Or 1280xwhatever in 4:3 ratio case. Then use ffmpeg or other tools (www.thebest3d.com/ffmpeg for examples) to convert & resample down.

Youtube and others (Animato.com) can accept high-res clips and feed them in standard as well as HD with the click of a button by the person viewing it.