I am using my first VirtualBox VM in order to run Windows 8 Preview on my Windows 7 machine. It looks like only a few resolutions are available for me to choose. My physical display is 1600x900, so I'd like to be able to set my Windows 8 VM to that in order to have a fullscreen experience. How do I add a 1600x900 to the choices I have in the "Screen Resolution" dialog?
Sounds like all you need to do is install the "Guest Additions" inside your guest OS (which would be Windows 8). It's pretty easy since it's Windows. Just press your host key (probably the right CTRL key) and D. This will mount the Guest Additions .iso as if inserting a CD into a real drive and run the guest additions installer while you're in Windows 8. When done, just reboot the guest (Windows 8) and you should be able to resize to whatever resolution you want. And to go full screen, press the host key (again, probably the right CTRL key) and F to toggle between full screen and a normal window.
There are other ways to do this but that would probably be the easiest. Hope it helps.
You can use
This solution worked once for me but now that I have 2 physical monitors it is not working.
You can set the video mode on a running VM by using the following virutalbox command
"c:\Program Files\Oracle\VirtualBox\VBoxManage.exe" controlvm "YOURVMGUESTNAME" setvideomodehint 1920 1200 32
VBoxManage controlvm "YOURVMGUESTNAME" setvideomodehint 1920 1200 32
This setting will take effect immediately and sometimes survive reboot.