How to add a new screen resolution size in ubuntu ?

I have installed VM in my machine and running Ubuntu in that. I have a laptop with resolution 1600x900 but inside VM in ubuntu it was not coming up as default resolution.

To check all the available resolution I did something like this :-

the xrandr command to check available screen resolutions.

⚡ xrandr
Screen 0: minimum 1 x 1, current 1440 x 900, maximum 8192 x 8192
Virtual1 connected 1440x900+0+0 (normal left inverted right x axis y axis) 0mm x 0mm
   800x600        60.0 +   60.3  
   2560x1600      60.0  
   1920x1440      60.0  
   1856x1392      60.0  
   1792x1344      60.0  
   1920x1200      59.9  
   1600x1200      60.0  
   1680x1050      60.0  
   1400x1050      60.0  
   1280x1024      60.0  
   1440x900       59.9* 
   1280x960       60.0  
   1360x768       60.0  
   1280x800       59.8  
   1152x864       75.0  
   1280x768       59.9  
   1024x768       60.0  
   640x480        59.9  
Virtual2 disconnected (normal left inverted right x axis y axis)
Virtual3 disconnected (normal left inverted right x axis y axis)
Virtual4 disconnected (normal left inverted right x axis y axis)
Virtual5 disconnected (normal left inverted right x axis y axis)
Virtual6 disconnected (normal left inverted right x axis y axis)
Virtual7 disconnected (normal left inverted right x axis y axis)
Virtual8 disconnected (normal left inverted right x axis y axis)

So here my system is not supporting the desired resolution. So i was googling around to get some better solution and found something like this :-

⚡ xrandr --newmode "1600x900_60.00" 118.25 1600 1696 1856 2112 900 903 908 934 -hsync +vsync

⚡ xrandr --addmode Virtual1 "1600x900_60.00"

⚡ xrandr --output Virtual1 --mode "1600x900_60.00"

⚡ 

And it worked like charm :-)

Source :- http://ubuntuforums.org/showthread.php?t=1966900