Really varies, technically larger the image the better the overall quality will be. Backstretch has a specific purpose though, are you rotating images by chance? You could just set the background on Style properties -> Page setup -> Page background if you're using a single image, it gives you more control for mobile use as well.
A problem with backstretch on the entire page background, depending on the device it will skew the image to make it fit, the square image on a phone will look awkward. That's why I mentioned on full backgrounds I'd almost recommend just using the page background option in XF.