Can't set a higher screen resolution in a external display in a Dell Mini 10v laptop

The same occurs to me. You must add the resolution with:

xrandr --newmode <ModeLine>

To know the ModeLine of your resolution, do this:

$ cvt 1650 900

You will get some like this:

# 1656x900 59.93 Hz (CVT) hsync: 55.98 kHz; pclk: 122.25 MHz
Modeline "1656x900_60.00"  122.25  1656 1752 1920 2184  900 903 913 934 -hsync +vsync

Then, copy the information after the word “Modeline” into the XRandR command:

$ xrandr --newmode "1656x900_60.00"  122.25  1656 1752 1920 2184  900 903 913 934 -hsync +vsync

After the mode is entered, it needs to be added to the output:

xrandr --addmode VGA1 1656x900_60.00

Now you have the new resolution :-)

FelGutiCo and milkovsky are right, however I had to use the -r parameter to make it work (otherwise the left 20% of my monitor was blank and the image was fit into the right 80%), so what I did was:

$ cvt -r 1920 1080
# 1920x1080 59.93 Hz (CVT 2.07M9-R) hsync: 66.59 kHz; pclk: 138.50 MHz
Modeline "1920x1080R"  138.50  1920 1968 2000 2080  1080 1083 1088 1111 +hsync -vsync

$ xrandr --newmode "1920x1080R"  138.50  1920 1968 2000 2080  1080 1083 1088 1111 +hsync -vsync

$ xrandr --addmode VGA-1 1920x1080R

Plus one hint, as I had to change "VGA1" to "VGA-1"; to find out the IDs of my monitors I used:

$ xrandr --listactivemonitors

$ cvt 1366 768
# 1368x768 59.88 Hz (CVT) hsync: 47.79 kHz; pclk: 85.25 MHz
Modeline "1368x768_60.00"   85.25  1368 1440 1576 1784  768 771 781 798 -hsync +vsync

$ xrandr --newmode "1368x768_60.00"   85.25  1368 1440 1576 1784  768 771 781 798 -hsync +vsync

To find id of monitors

$ xrandr --listactivemonitors

Monitors: 2
 0: +*eDP-1 1366/345x768/194+0+0  eDP-1
 1: +DP-1 1024/271x768/203+1366+0  DP-1

$ xrandr --addmode DP-1 1368x768_60.00