The body of the HTML document does extend infinitely in every direction. However if you want to restrict your background to one position you can have it not repeat by using styling. You can also position your background.
Here’s some css code you might use:
<style type="text/css">
body {
background-image: url(yourbackgroundimage.jpg);
background-repeat: no-repeat;
background-position: center;
}
</style>
If you’re looking for a page that appears to have a set width you can embed all your content into a table, div, or some other HTML container that has a set width and set the background image of that element to hold your background.
Abraham