Photoshop - Opening a File
JJMacks has given you the best method.
You can find a discussion and a link that will download a fit-to-screen script with instructions here.
The quick and dirty method is just to hit Ctrl + 0 to fit to window. On a Mac that would be Cmd + 0.
Living and loving it in Bangkok, Thailand. Canon 7D - See the gear list for the rest.