The size of elements on the screen are controlled by the computer's desktop resolution. When using Windows 7, right click on any empty space on the desktop to bring up a menu. Click on Screen Resolution to change the settings.
On other Windows systems, go into the Control Panel and find Display. Choose Screen Resolution to resize the screen. The larger the numbers in the resolution options, the smaller the text and icons appear. If the resolution is as high as the numbers allow, there may be an issue with the video card drivers. Reinstall the drivers, and see if the screen size returns to normal.
A program called Magnifier also has the ability to change the size of the computer screen. If a faint magnifying glass appears in the top corner of the screen, then Magnifier is enabled. Close out the program to turn off the magnified effect on objects on the desktop.