If you really want very tiny icons, right click desktop, properties, appearence, advanced. In the window click on the minimize/maximize/close buttons and reduce the size. This setting will make icons smaller wherever they are displayed as small icons, quick launch and other toolbars, start menu programs, titlebar. Don't think it is possible to change the size for quicklaunch only. It will not effect the min/max/close buttons as long you don't use Windows classic theme.
If you should insist on a registry tweak you can directly make the changes in
HKEY_CURRENT_USER\Control Panel\desktop\WindowMetrics Caption Height
THIS WORKS BEST!
right click on the task bar make sure it is NOT locked (no check mark on "lock the task bar"
right click in the area between the start button and the the icons your cursor should turn into a double arrow
Change the to small icons.