Advice for websites with a large background image

Anyone has done any pages with large background images like: the old squareup page? (can't link to image)

I am wondering if anyone has any advice or point to some nice templates? How large should the background image be if you want it to take up the whole page (both px size and file size) Any advice or links to a good template would be welcomed.

this is a jquery method full page background image http://css-tricks.com/examples/FullPageBackgroundImage/jquery.php
just click the button at the bottom to get the source code