How to reduce the Quick Launch bar icon size in Windows Vista?

You have to uncheck the "Lock the Taskbar" first in order to reduce the Quick Launch bar icon size. To reduce the Quick Launch bar icon size, perform the following:

1. Right click on an empty area in the Taskbar and uncheck the "Lock the Taskbar" option if it has a tick mark Clicking the "Lock the Taskbar" again and again will make the Taskbar to toggle between the unlock and lock states).
2. Right click on the Quick Launch area and click "View" and click "Small Icons".

To lock the Taskbar again, right click on an empty area in the Taskbar and check the "Lock the Taskbar" option.