GIMP has [Image][Print Size] that can scale a picture, but you may still need to play with the printer dialog to make sure it does not change the size. On my printer there is an option to scale, and I set it to 100% (it normally scales to 97% to avoid margins).
Another way that seems to work every time is to use Word. Open a blank document, us the [Insert] menu and the [Picture] button to get the picture in the document, then right click on the picture to see its dimensions. You can type in any size for Horizontal or Vertical and it will resize the picture. Then when Word prints it, it comes out just that size.