For the "still image" assignment I asked you to turn in a 3000x2400 pixel image (high resolution, for printing up to 8x10 inches at 300 DPI). (Or you could do a 2400x3000 pixel image, if you want portrait orientation, instead of landscape).
During testing, I would use a smaller resolution (maybe 1000x800), so you can try things and get results much faster, but once you have your final scene, lighting, & camera angle all picked out, you should then use:
raytracing:output-resolution 3000 2400
or, for tall images:
raytracing:output-resolution 2400 3000
However, for a video, this high resolution will probably take forever to render, so maybe something more like 800 x 600 would be a good choice.