[balinor]

If you have a look at the Template Editing Guide, you'll see that rectangle_top.tpl controls the width and position of your site. I usually get rid of the wrapper table and turn it into a <div>, and then you can use CSS to style the container. If you stick with the table, you simply change the width and add a center tag to it. The logo is in head.tpl, which contains a bunch of nested tables you'll need to remove in order to get it centered.