Ubuntu 12.10 in Virtualbox 4.2.4 Screen resolution limited to 1600x1200

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http://askubuntu.com – I have installed Ubuntu 12.10 (x64) in VirtualBox 4.2.4 on Windows 7, and installed Guest additions with no problems reported. I set the display memory to 128Mb. After rebooting, the display settings only show 4x3 aspect ratio with a max of 1600x1200. My screen is 1920x1200. Have I missed some step in getting Ubuntu to use the full screen? I have this working with Ubuntu 11 (x86) but need a 64-bit installation. (HowTos)