Virtualbox complete fullscreen
I would guess that you have not installed the guest additions in your guest system.

Quoting from the manual:


  In the "Devices" menu in the virtual machine's menu bar, VirtualBox
  has a handy menu item named "Install guest additions", which mounts
  the Guest Additions ISO file inside your virtual machine. A Windows
  guest should then automatically start the Guest Additions installer,
  which installs the Guest Additions into your Windows guest. Other
  guest operating systems (or if automatic start of software on CD is
  disabled) need manual start of the installer.