How much space a photo will occupy on your screen depends on two factors:
1. Your screen resolution. You can find this by right click on your desktop,
click on Properties and click on the Setting tabs. You will see two numbers,
say something like 1280 x 1024 pixels. If you have a photo of these exact
dimensions, it will fit the screen. If exactly double, the software will fit
it in, if exactly half it will occupy half the screen.
2. The aspect ratio of your screen. This is simply the width divided by the
height. In the above example 1280/1024 = 1.25. So if you have a photo of
1280 x 1280 (a perfect square), even if you have more than enough pixels, a
square cannot fit into a rectangle. The software will then display this
photo with white or black space around it.