I tried to resize my display and discovered the mode I wanted was not available. Interestingly, the current mode was also not listed but that is a separate issue. Adding a new mode is not as straight-forward as I had hoped, so it is worth making a note (this post) of how to do it.

First thing needed is a mode-line, which can be generated with the cvt command (in this example for 1720x1440px at 60Hz refresh rate):

$ cvt 1720 1440 60
# 1720x1440 59.94 Hz (CVT) hsync: 89.48 kHz; pclk: 209.75 MHz
Modeline "1720x1440_60.00"  209.75  1720 1848 3032 2344  1440 1443 1453 1493 -hsync +vsync

With that mode-line, we can add the mode to xrandr:

xrandr --newmode "1720x1440_60.00"  209.75  1720 1848 3032 2344  1440 1443 1453 1493 -hsync +vsync

Before it can be used, the mode needs to be added to the output (HDMI-2 in my case):

xrandr --addmode HDMI-2 "1720x1440_60.00"

Finally(!), the mode can be used:

xrandr --size 1720x1440