This looks like a configuration problem on your server. NoMachine will only be able to set a resolution among the supported ones. If you are not able to set the given resolution even with the system xrandr command, NoMachine won’t be able to set it as well.
You may try with the following command:
xrandr –output Virtual1 –mode 1600×1200
You may also check what resolutions are visible in the system settings menu, and if it is possible to set the wanted resolution there.