How to change the screen resolution in Sun's virtual box

Make sure that the assigned graphics memory to the virtual box is at least 12MB. This you find in the configuration of virtual box -> General -> Basic.
Go to "Devices" and choose "Install Guest Additions"
Boot up Ubuntu
Change to the cdrom directory cd /media/cdrom0
Run VBoxLinuxAdditions.run as sudo
```
sudo sh ./VBoxLinuxAdditions.run
```
This creates the file /etc/X11/xorg.conf.

Modify /etc/X11/xorg.conf so that it looks like this:

Section "Device"
        Identifier      "Configured Video Device"
EndSection

Section "Monitor"
        Identifier      "Configured Monitor"
EndSection

Section "Screen"
        Identifier    "Default Screen"
        Device        "VirtualBox graphics card"
        Monitor       "Generic Monitor"
        DefaultDepth  24
        SubSection "Display"
                Depth         24
                Modes         "1280x800" "1024x768"
        EndSubSection
EndSection

You can insert any mode you like in "Modes".

Restart X by pressing CTRL+ALT+BACKSPACE.

References

blogs.sun.com/paulkasper/entry/customizing_opensolaris_screen_resolution_running