The show image in original size tool is located in the toolbox toolbar. When pressed, images are displayed in actual size. When not pressed, images are displayed in pixel size. Click on this tool to toggle between the two modes (or click Tools/Zoom).
For this feature to work correctly, the monitor and the images in the study need to be calibrated. You are automatically prompted to confirm monitor calibration. You also are prompted to define a selected image if it does not already contain pixel size and spacing information. If you apply image calibration settings to the study, you will not be prompted until you open another study.