I was trying to find the answer myself and just about gave up when I accidentally stumbled upon a fix. What I did was I unchecked the "Fullscreen" option, then dragged the window over to my main monitor, then went back to the main menu without saving the settings. Since I didn't save the windowed setting, it then went fullscreen on my main monitor and I was able to go back to the options and change the resolution to fit my main monitor.
Hope this fix works for you as well!