If you've uploaded a banner image in the Template Designer, but it's showing up too small when you view your pages on the front end, the first thing to check, is that the image you've uploaded is the optimal 940px wide and 72 pixels/inch. That will make sure it's the proper size to fit in the header location.
If you've confirmed that your image is in fact the actual size, and is getting "shrunk" and displaying much to small, please open a ticket with our support team and we will be happy to help you out. Sometimes, the styling for that element in the Standard Template can get confused and we'll be happy to manually fix that for you.