ubuntu resolution

cvt 1900 1200

creates modline

xrandr --newmode 1900x1200 63.50 1900 1072 1176 1328 1200 771 775 798 -hsync +vsync

xrandr --addmode VBOX1 1900x1200

Since above command is not permanent sollution to add new resolution, you need to edit /etc/gdm/PreSession/Default
Enter followings at the end of file /etc/gdm/PreSession/Default

1. xrandr --newmode "1240x1024_60.00" 105.50 1240 1320 1448 1656 1024 1027 1037 1063 -hsync +vsync
2. xrandr --addmode VGA1 1240x1024_60.00