Jameson is definitely on the right track. The trick is the canvas size needs to be the same as the "printable area" of your paper.
For example, I've been trying to print an image to very specific dimensions, 392mm x 160mm. I know this needs to be printed on A3 paper but setting the canvas size of the image to A3 dimensions does not give me the right image size. It's off by a few mm. I discovered the printer has hard print margins of 7mm on the leading edge and 2mm on the other edges. If I reduce the canvas size to account for the print margins, 420mm-7mm-2mm x 297mm-2mm-2mm = 411mm x 293mm, the image dimensions are perfect. Print using the defaults on the print dialog.
