To add a higher screen resolution in newer versions of Ubuntu/Debian that don't use the old /etc/X11 configuration files do this:

1. Open a terminal session as root.

2. Use your monitor manual or find info online that gives your monitor's highest resolution setting.

3. From the command line, type:   xrandr
    This will give you information similar to this: Screen 0: minimum 64 x 64, current 1024 x 768, maximum 32000 x 32000
VBOX0 connected 1024x768+0+0 0mm x 0mm 1024x768 60.0*+
800x600 60.0
640x480 60.0     Take note of what the word in red, is. You will need this in a minute.

4. Now type:   cvt hhhh vvvv   where "hhhh vvvv" are the screen resolution amounts. (Don't put an "x" between them.   E.g. "cvt 1024 768")
    This will give you information similar to this: 1024x768 59.92 Hz (CVT 0.79M3) hsync: 47.82 kHz; pclk: 63.50 MHz
Modeline "1024x768_60.00" 63.50 1024 1072 1176 1328 768 771 775 798 -hsync +vsync

5. Copy the second line of output from the previous step and add it to the end of this command:   xrandr --newmode
     (e.g. xrandr --newmode Modeline "1024x768_60.00" 63.50 1024 1072 1176 1328 768 771 775 798 -hsync +vsync )

6. Now add this new resolution to your resolution options with this command: xrandr --addmode VBOX0 1024x768_60.00    where the red name is what you got from step 3
and the purple resolution name is what you used inside the quotes in step 5.

7. Now from your GUI desktop go to the Sytem Settings, open the Display Settings and you should have your new resolution options listed as the name you used in purple above. Pick that one and save the change.