Importing an image as a canvas to sketch on and setting its size properly

Hi,

I would like to import an image and set its size so I can sketch over it with real size.

For now, I can easily import an image right on the working grid. I can scale it arbitrarily, but I would like to scale it to a precise dimension so that a 5 inches segment in my image would really measure 5 inches. I would then sketch over it and my sketch would have real size dimensions.

Thanks

JF.