In the Ubuntu Unity interface, Launcher is the taskbar that holds the icons of the programs that are currently running or pinned to it. If you want to maximize usage of the desktop space, you may find the space occupied by the launcher and its icons to be too much. The width of the launcher is tied to the size of the icons.
Reducing or changing the size of Launcher or its icons is simple: go to System Settings > Appearance > Launcher icon size and change it. The default size is 48. I find that the smallest size of 32 is enough for my needs.
Tried with: Ubuntu 12.04 LTS