do you mean putting a background image on a website that fills the entire screen without repeating? it's impossible to say, because everyone has different monitor sizes.
a safe resolution to use for large backgrounds is 1280x800, as that's what a lot of people have these days. basically, what you can do is put your image in the center of a file that size, then gradient fade the edges to a solid color. your code would look something like this:
CODE
body { background:#[HEX COLOR OF SOLID BACKGROUND] url(LINK TO IMAGE) top center no-repeat fixed; }