Image size on screen is usually determined by the number of pixels. If you have your screen set to 600 X 800 and the camera is taking 1200 X 1600 DPI pictures you obviously wont fit all the pixels on the screen. Simple viewers are usually limited to that view.
I dont have Picture-It on my computer right now, but I know it will make the display fit the screen so you can work on large images. You dont want to actually reduce the size of the image as that makes a lower resolution. Save all of the pixels you have.
If you are just seeing it too large in your default viewer I suggest you load Irfanview. It can be set to show the image so it fits on your screen without actually reducing the image size:
http://www.irfanview.com/english.htm Let it take over as the default viewer for all of your image types. It opens quickly and is very capable in printing etc. You can always open Picture-It and use File>Open to open any image you want to work on.