How can I fit the Ubuntu desktop to the screen in VirtualBox?

You need install the virtualbox guest additions:

  • In the VM's menu: click "Devices" -> "Insert Guest Additions CD Image" (The guest additions cd image will be mount in your guest's cdrom"
  • Run

    .{path_to_your_CDROM_mount_in_the_GUEST}/VboxLinuxAddition.run

  • Restart your VM

  • Now, with the correct driver installed and loaded: "You have to select "Auto-resize guest display" on "View" menu".

Hope this help!