Recently I have been keen to try the Virtualbox Seamless desktop mode, which allows you to run a virtual machine and have it integrate with your host OS’s window manager.

After installing it, removing the default Guest additions on Mandriva 2008.1, and using the updated GA, I was unable to select any screen resolution higher than 1024×768. I’ve found lots of forum posts asking about how to fix this problem of Virtual box limiting the max res, but not many answers (apart from checking xorg.conf).

The answer lies here in the 1.6.4 changelog

Linux additions: accept every user-defined guest video mode in /etc/X11/xorg.conf

After updating my host machine (windows) and the guest additions to 1.6.4, I am able to select 1280×1024.

If this doesn’t work for you, make sure you have the latest Guest additions installed.