These three icons control the size of the picture. Best fit makes it always fit within the current window size. You can make it show actual size (100% of the original) or make it any pixel size or zoom you like
Best Fit. Key shortcut: Ctrl+B
Always makes the picture fit within the current window size. Resize the window by using the grab handle in the lower right corner.
This is the normal state when a new picture is opened.
Actual Size. Key shortcut: Ctrl+A
If you select this option the picture is shown at actual 100% size. In other words each pixel of the image is shown as one pixel on the display. This is sometimes referred to as 1:1 size.
In the case of high resolution images from scanners or digital cameras this shows the full detail of the image.
Size/Zoom'. Key shortcut: Ctrl+R
With this tool you can adjust the size of your picture these ways:

The following key-short cuts work at all times (number keys above the keyboard):
1 - zoom to 100% (Actual size)
2 - zoom to 200%
3 - zoom to 300%
4 - zoom to 400%
5 - zoom to 50%
Numeric keypad + Zoom in (double size)
Numeric keypad - Zoom out (half size)
Notes