Print size

I'm a total neophyte at Gimp. I have been unable to print any image at any size other than 35mm X 55mm. I have tried "Image Size", "Scale Image" and every page setting I can think of. I have scanned a print I want to use, attempted to manipulate it, saved it as a Gimp image, jpeg and attempted to reuse it. I have also uninstalled and reinstalled Gimp -- all with no change.

I'd appreciate any assistance available.