use the EXPORT feature instead of print ... export your board at 600 or 1200 dpi (whatever the res of your printer is)
this will give you a rather large bitmap, but it will be at the exact scale you need it to be, then you just use any picture viewer app and print the picture without any scaling options selected ... should come out exactly right.
my method (for making transparencies for the photolithographic method) is a bit more involved ... I go to export, 600 dpi to clipboard ... then I fire up paintshop pro, paste my design as a new image ... now I clean up any weirdness (like tiny orphans of a ground plane pour), I also mirror the design if needed, and then copy and paste it into a new image, sized for the PCB blank i'll be using ... I can usually fit 6-12 designs on a single 4x6 inch PCB. Then I save the file in the TIFF format, so the scale is preserved ... now I open it on my machine with the inkjet printer, print it to acetate and off I go.