When you only define one widget area (that's what you get when you use the minimal options) the widget area is the entire with of the section it is placed in. Since you have defined a widget in the header area, the widget area spans the entire width of the page.
But you want it centered and not spread across the page!! How do you do this? The answer...CSS!
You can use CSS on the new widget areas just like everywhere else, so lets say you added the following to the header area
HTML Code:
<?php bfa_widget_area('name=Header_Widget'); ?>
(NOTE: I named the widget with an underscore to make it easier to address it later)
And you want the widget to be in the center and only take up 30% of the page width, here is what you would code as your CSS Insert:
HTML Code:
div#header_widget {
width: 30%;
margin-right: auto;
margin-left: auto;
}
this will keep the widget centered and only take up 30% of the page space no matter how wide it is. You may want to make this a fixed width depending on what you are using the widge for. Have fun.