I have an Acer VGA monitor, resolution 1600 x 900.  The last time I reinstalled Mint, the resolution wasn't detected.  Mint 17 couldn't do it, Ubuntu 16.04 couldn't do it; Windows 8.1 worked almost flawlessly (almost, because some of my attempts to fix the problem messed up the display on the windows side).  

This was fixed a while ago, but I'm making a record of the solution.  I created a small file called set-screen.sh, put it in my home directory, and added it to my startup programs.  It would automatically reset the resolution of my external monitor.  Hackish, but it worked.  

#!/bin/sh
sleep 7

xrandr --newmode "1600x900_60.00"  118.25  1600 1696 1856 2112  900 903 908 934 -hsync +vsync
xrandr --addmode VGA1 1600x900_60.00
xrandr --output VGA1 --mode 1600x900_60.00

The sleep 7 command ensured that my computer had a chance to turn on completely and actually detect the external monitor before running the command to modify the display.  I have a pretty fast computer; the recommended sleep time was actually 15 seconds.