When you are setting up your site and you want to place an image behind some content or to cover a whole area it is import to understand a few key options in terms of background size. Here is a quick tutorial outlining the important things to grasp when setting up your background.
Now a background image is just that it’s a background so you cannot set the size of an area by adding a certain size background image. The size of whatever you’re adding this background image to will be determined by the content that is within the container. What you can determine with a background is how that image fills the space of the container.