User Configuration > Policies > Centrify Settings > Mac OS X Settings > Dock Settings > Adjust the Dock’s icon size
Set the approximate size of Dock icons in pixels. The valid settings for the Dock size range from 16 pixels (small) to 128 pixels (large). The default size is 80 pixels.
Note: This setting is approximate because the actual size of Dock icons depends on screen resolution and the number of icons in the Dock.
Once enabled, this group policy takes effect when users log out and log back in.