Here's an xorg.conf for virtualbox (1024x768)

[seamuso]

You have to install the vbox additions for it to work. I'm using it with the most current close sourced version (need the usb support).

# xorg.conf (X.Org X Window System server configuration file)
#
# This file was generated by dexconf, the Debian X Configuration tool, using
# values from the debconf database.
#
# Edit this file with caution, and see the xorg.conf manual page.
# (Type "man xorg.conf" at the shell prompt.)
#
# This file is automatically updated on xserver-xorg package upgrades *only*
# if it has not been modified since the last upgrade of the xserver-xorg
# package.
#
# If you have edited this file but would like it to be automatically updated
# again, run the following command:
#   sudo dpkg-reconfigure -phigh xserver-xorg

Section "InputDevice"
	Identifier	"Generic Keyboard"
	Driver		"kbd"
	Option		"XkbRules"	"xorg"
	Option		"XkbModel"	"pc105"
	Option		"XkbLayout"	"us"
EndSection

Section "InputDevice"
	Identifier	"Configured Mouse"
    Driver      "vboxmouse"
    Option      "CorePointer"
EndSection

Section "Device"
	Identifier	"Configured Video Device"
	Driver		"vboxvideo"
EndSection

Section "Monitor"
	Identifier	"Configured Monitor"
	Option		"DPMS"
EndSection

Section "Screen"
	Identifier	"Default Screen"
	Monitor		"Configured Monitor"
	Device		"Configured Video Device"
	DefaultDepth	24
	SubSection	"Display"
		Depth	24
		Modes	"1024x768"
	EndSubSection
EndSection

Section "ServerLayout"
	Identifier	"Default Layout"
	Screen		"Default Screen"
EndSection

[gymophett]

Thanks man! This saved me a whole lot of crap.

Just change the mode to your desired screen resolution and your good to go.