Krazykickz,
In the current

Commerce software to make your site expand out to the fullscreen of the browser window do the following:
1. Log into your admin panel
2. Go to Site Html - Top of Page Editor. At the top of the screen there will be an area for pixel width or percentage width. Make the screen width set to percentage and make the numeric amount 100. This way your site will always be 100% and expand to the full width of your browser.
At

Commerce Design we design all of our sites for 792 pixels wide. But usually when we go to integrate them into the stores we set up the sites to be 100% width. This way the sites are always full screen.
Sometimes you may want to make your site a fixed width and not a percentage width for asthetic reasons. The primary concern against this is that you have to be very meticulous with the rest of your site meaning you cannot have any of your product pictures larger than 300 pixels wide or your middle area of your site (left, center & right columns) will balloon outside of the top and bottom portions of your site.
Let me know if this makes sense