The problem is that the image is larger than any viewport so FancyBox will try to make it fit independant of what default size you set. Is there an option on your Settings > Media admin page that says:
Scale large content down to fit in the browser viewport.
If so, uncheck that option. If not, upgrade to the latest development version.
This way, you can make the image go full size but you cannot force bare images to be show in a certain size. However, this can be done for inline content by unchecking the option:
Try to adjust size to inline/html content.
But you will need to include your image as inline content instead of as a simple link...