Full Screen Mode

When I play my slide show in full screen mode on a second monitor the images are enlarged to fit the screen size. This is for photograph judging and the images must be displayed at their original size and resolution. I have set the stage size and the custom export size to the required image size (they are all the same size) but the images are enlarged on the display monitor no matter what I do. How can I display images on the second monitor without enlarging the images? I use teleprompter mode. Thanks for any help!