Jake,
As you indicated that it only takes up 50% of the width, I'd recommend that you look at the Gantry template settings under layout to see if the 'header' position is set for 2 modules with 50% width on each. If so, you need to change it to '1' module position and it should default to 100% width. As far as forcing a set width / height, I usually embed my header images inside a DIV with an ID so I can do custom styling to adjust for margins, padding, borders, etc.
Home page example:
HTML in module 'header-home' published to 'header-a' position. Note: Template setting has 'header' position layout set to 1 module and 100% width. Also, there are 'two' images but it could easily be just one.
<div id="header-home"<div id="header-home-left"><img src="images/modules/header/home/header-home-logo.jpg" alt="Home Page" height="125" width="740" /></div><div id="header-home-right"><img src="images/modules/header/home/header-home-address.jpg" alt="business address" height="125" width="200" /></div></div>
Then I use CSS in the custom stylesheet to specifiy margins and padding to align the images as well as the "float-left" code for the 'header-home-left' DIV. Please note that although I have put the height and width inside the image html code, I usually don't put it there as I like to specify the width and height in the CSS for good coding practices. I've shown it here for example purposes only.
Now some people might look at this code and say, why not set the layout positions for '2' modules and assign a fixed width to each? The reason is flexibility. On some pages, I might want the home page 'header-a' module to have a 'logo' image that is 740 pixels wide and the side image to be 200 pixels wide whereas on another 'header-a' module assigned to other links, I might want the logo image or content to only be 470 pixels wide and the side image or content to be 470 pixels wide with CSS setting margins and padding.
If you have 50 links and you have three different header 'styles', you could duplicate the template, assign to the links you want, change the layout settings for the 'header' position and use the fixed widths for the module positions. This procedure will work if the widths fit your requirements. I've done it this way on a couple of sites and it works just fine.
Luke