Make VirtualBox use full screen size in Windows 8

1. Close your virtual machine.

2. Navigate to C:\Program Files\Oracle\VirtualBox\.

3. Hold Shift and right-click in the blank space of the window.

4. Select Open Command Window Here.
   
   

5. Type VBoxManage.exe setextradata "W8 VM NAME HERE" CustomVideoMode1 1920x1200x32.
   
   

6. Restart your VM. Now you will be able to select 1920x1200 resolution.
   
   

With my setup, the installation of the Guest Additions by hand was required. Even requesting it from the UI didn't cause Win8 to see or run the installer.

Folks on a Windows host can use the steps here, but for someone on a non-windows host OS, you will need to drop the VBoxGuestAdditions.iso into the Virtual Media Manager so it gets mounted as an Optical disk.

In my case, it was located (as described) in /Applications/VirtualBox.app/Contents/MacOS

From there, the shortest distance for me was to go to the desktop, then open the library folder in the bottom left, and navigate first to the computer (in the sidebar) and then to the Optical disk.

The installer itself ran correctly in Win 8 and prompted for a reboot. Once the guest OS rebooted entering full screen mode and waiting several moments for the drivers to scan the display did the trick. Win8 correctly adjusted things without any need to open the screen resolution Control Panel.

You need to ensure that the guest has sufficient video memory to go full screen. You might also want to enable 2D/3D acceleration. When there is enough, switching to fullscreen will work as usual. Settings: