Updated July 16 2012, this was done on Lubuntu 12.04 and VirtualBox 4.1.18

My Setup, 24 inch LG flat screen with 1920x1080 resolution, Wanted to play around with new ubuntu 10 workstation, Installed Ubuntu but defaulted to only 640x480, 800x600 and 1024x768.

So this is the steps I had to take to have more options on video resolutions.

Bring up a Terminal window,

Xrandr  is  used  to set the size, orientation and/or reflection of the outputs for a screen. It can also set the screen size.

Type xrandr and you will see the resolutions setup, the second line is important because it tells you the name of video adapter.

Cvt calculates VESA CVT (Coordinated Video Timing) modelines for use with X.

# cvt 1440 900 and copy everything after Modeline

You could use other resolutions, see below:

1920x1080
1680x1050
1280x1024
1152x864
1280x720
1024x768
832x624
800x600
720x480
640x480
720x400

Adding the new mode
# xrandr --newmode "1440x900_60.00" 106.50 1440 1528 1672 1904  900 903 909 934 -hsync +vsync

# xrandr  --addmode VBOX0 1440x900_60.00

Test the new resolution right away
# xrandr --output VBOX0 --mode 1440x900_60.00

If you are happy with this resolution add information to Default file so after reboot it will be there.
# nano /etc/gdm/PreSession/Default

Add theses two lines to the bottom of the file. (I didn't need to do below for Lubuntu 12.04)

# xrandr --newmode "1440x900_60.00"  106.50  1440 1528 1672 1904  900 903 909 934 -hsync +vsync
# xrandr --addmode VBOX0 1440x900_60.00

Save and restart your system, the first time you login you have to set your prefered resolution and then it will be set all the time after that. Go to System > Preferences > Monitor and choose the new resolution.

Found some of the information at this website linuxreaders