I have always had issues with the screen resolution for my guest OS when using Virtualbox.
Solution: After installing the guest OS, Run "Guest additions" under "Devices" in the Virtualbox window. This will mount a CD-image in your guest. Run the appropriate file on that CD as root.
I also had to edit my xorg.conf
gksu gedit /etc/X11/xorg.conf
And ad a line:
Section "Device"
Identifier "Configured Video Device"
Driver "vboxvideo"
EndSection
(Italic line added).
Restart and everything worked great.
This is a good link for more info
