Eagle - How to limit exported images to board layout?
At this moment I don't have my PC with EAGLE so i can't tell you the exact procedure, but when I had to export my layout to check the components size, I printed it with real size. I didn't export image i directly printed it.

If you send it to PDF creator or similar programs the image sizes may change and this will affect your case.
Send top and bottom separately or print on two sides of the sheet.

Hope this will help you.