I had a slightly different issue with purchased version.
Every time I ran the game in Windowed mode with Screen/Window size as Desktop, I would get a window full width, but only (and exactly) half height.
I had locked taskbar, I had un-checked auto-hide, Borderless Gaming didn't help... I was baffled. Then I did something completely silly, using the sort of logic inspired by Vaughan's take on Alexander Pope's words
: "To err is human; to really foul things up requires a computer."
I moved the perfectly-configured taskbar from monitor 1 on the left, used for my browser and chat and temperature gauges and such, to monitor 2 on the right, slightly closer to me, which I use for gaming (and thus an inappropriate place for the taskbar, given that windowed games may get confused by it, or at least may have their bottom dozen pixels covered up by said taskbar).
VOILA!! By moving the Taskbar to the same window as the game is intended to run on, it worked! Desktop sized window! Glory of glories, finally Steel Panthers covering my screen, allowing my to make war without squinting!!
As soon as the game window properly draws, I was able to unlock the taskbar, move it to the other window, and set it to auto-hide again. Also note, simply moving the "winSPWAW Game Options" window to Monitor 2 did not
resolve the issue. The taskbar had to be moved to the monitor on which I expected the game to play on... even though 2 is identified by Windows as my Main Display. I can't fault any programmer here, some of this visual/display/etc API programming is like drinking a pan-galactic gargle blaster (and if you don't get that, for God's sake go read you some Hitchhiker's Guide).
So in short, in a multi-monitor setup, make sure the taskbar is locked, auto-hidden, and on the same monitor as you want to run the game on
when you fire up the game! Then set it however the devil you want afterwards.
Hope this helps someone!