html, body {
min-height: 100%;
}
Will do the trick.
By default, even html and body are only as big as the content they hold, but never more than the width/height of the windows. This can often lead to quite strange results.
You might also want to read http://css-tricks.com/perfect-full-page-background-image/
There are some great ways do achieve a very good and scalable full background image.