Larger Logo in Aqua Theme ?

I would like to use a larger logo (more like a banner) that stretches across most of the white header area in Aqua; it seems that when I upload the image which I would like to use, it is automatically resized (smaller). Is there a way to use an image which is, say 600×90 ? It has the client company’s logo to the left, and the name and slogan to the right, hence the banner-style shape. Do I need to add some custom CSS to make the “eight columns header_left” wider ?

Thanks,
Colin