The carousel in new themes is located just below the navigation, and is as wide as the navigation. So it spans the main content and any side navigation menus that are enabled. If you would like to have the carousel moved into the main body, you will need to use a new template. There is a page template that has already been created that puts the carousel into the main body instead of above the main body. You can download this page template, then upload it into your ShopSite store under Merchandising > Custom Templates > Pages. Then go to your list of pages > select your home page > Edit Page Layout, and change your home page template to the BB-InnerPageCarousel.sst, and publish changes.
Download Template Here: http://shopsite.com/templates/cookbook/builtintemplates-bb.shtml
The width of the main content area is 82%, minus the side padding (which is 16 pixels on either side in most new themes). So if you main page width (Preferences > Layout Settings, see what your page width is set to) is 1000px, and you have 1 side menu enabled (just the left or just the right), then the side menu will be 18% of the page, and the main content will be 82% of the page, meaning the white area in the middle of the page is 820px wide. However, you then have to minus the padding, 32px (16 + 16), so the main content area where you can actually have text or images would be 786px wide (assuming I just did my math right
). So for best results (best load times and best image quality), you will want to have your banner images be 786px wide, and whatever height you want (but make the banner heights uniform).
If you upload banner images that are larger than 786 pixels wide, they will be forced smaller, which looks fine to the customer, but will take a little longer to load because they are larger images (unnecessarily large). If your banner images are smaller than 786 pixels wide, your images will be stretched to the width of the area available. In many cases, if your banner images don't have text on them, the images will still probably look ok, especially if you have a title or description specified under Preferences > Carousel, because the text will be the focus.