I haven't found a noticeable improvement using enormous backgrounds, if anything it slows things down and uses more memory. Some people might want to render a very high resolution image, and they would benefit from having more pixels, but for freestuff you have to think of the needs of the many; and download time is definitely going to be part of the equation. I suggested 1024 x 768 because it's my screen resolution; it's the size I usually render at; and I usually make my backdrops that size.