How to change desktop resolution for Sparky with LXDE


I have an Allo Sparky when attached to a monitor resolution is listed as 1920x1080 but the desktop is very small with large icons and the resolution as reported by xrandr is 1280x800.

I’ve tried changing the resoultion of the desktop using:
xrandr --auto --output HDMI-1 --primary --mode 1920x1080

I receive the message:
xrandr: Failed to get size of gamma for output default
warning: output HDMI-1 not found; ignoring

Any suggestions? Thanks for your help.