How To Readjust Image Size In Photoshop Cs5?

How do you readjust image size in Photoshop CS5? I selected medium-rectangle banner in Photoshop CS5 New Document Window pop-up but I need to readjust the image size because the image is too big. The image isn't fitting inside the medium-rectangle banner correctly. I downloaded a logo image from Google images and I'm having trouble figuring out how to resize it to fit within the dimensions of the medium-rectangle banner. I'm trying to make medium-rectangle banner and other various size banners to advertise my services in the small biz ads section on different classified ad sites. I need some guidance on making banners in Photoshop CS5. Thanks.