When you switch the Viewer into full-screen (toolbar top right), it will fill your entire screen.
The size of the image depends on the image size you or your IT people have configured for IMatch Anywhere WebViewer in the Settings.
Depending on the aspect ratio of the image and the aspect ratio of your screen, there will always be some vertical or horizontal space - the Viewer does not crop or stretch images to fill the screen.
For 100 fill rate presentation purposes, you would usually export images in the batch processor to the same size of your screen, using one of the fill modes in the Batch Processor to control how the Batch Processor crops the image in order to fill the requested size.