If you have tried running Windows 8 in Virtual Box (a great way to try out a new Windows version before taking the leap to install it as your main OS) you might have noticed that the only display resolutions available are in 4:3 aspect ratio.
To fix this, open a command prompt (PowerShell in my case) do this:
C:\Program Files\Oracle\VirtualBox> .\VBoxManage.exe setextradata “Windows 8 test” CustomVideoMode1 1600x900x24
“Windows 8 test” is the name of the virtual machine in VirtualBox and you can select any resolution you want. Restart the virtual machine if it’s running and after that the new resolution will be available in the Screen Resolution settings inside Windows 8.
Note that it’s possible to add several new resolutions to choose from by using CustomVideoMode1, CustomVideoMode2, etc. This can be useful if you want to move between monitors in multi-monitor setups. To do that in full-screen mode, press <Host>-Home which will show a popup menu where the display can be configured. <Host> is mapped to Right-Ctrl by default.
Credits for this tip:
Running Windows 8 on VirtualBox with Additional Wide Screen Resolution