Learn how to reduce the size of your Windows desktop icons if the video resolution is already on the highest settings.
Click the blank space of the desktop, hold down the Ctrl key and scroll down using mouse wheel. Scale the desktop icons until they’re fix your size.
Ctrl + Mouse wheel scroll works to zoom in most of the applications (web browser, MS Office, Notepad, Windows Explorer etc.)
Another Method 1:-
Right click on the Windows 7 desktop -> View -> Select Small icons or Medium icons which size you want.
Another Method 2:-
You can also change the windows 7 desktop icon size via registry settings.
HKEY_CURRENT_USER\Control Panel\Desktop\WindowMetrics] "Shell Icon Size"="36"
36 is a default (medium size) value.