1. Your screen resolution changed back to the 800 X 600 which would give your massive icons.
Go to System Preferences > Displays and change it back to "Best...."
2. You turned on Universal Access (Accessibility) accidentally.
Go to System Preferences > Accessibility and check that there are no check marks in the "Use keyboard shortcuts to zoom" section. (other than Smooth images)
Let us know how you get on
Not sure what I did but everything on my desktop suddenly is supersize, the icons on my desktop are big as well. I lost the image as a screen saver display. My desktop icons are not as the were and can not be rearranged. I have an IMac with 10.5.8. I have tried everything including a restart but did not help. Any suggestions ?? Bill