I have performed some more tests today.
Having only my 1920x1080p monitor plugged into my laptop the same issues apply, whether in windowed mode or non windowed mode.
Having unplugged everything from the laptop and just using the laptop's built in screen (17.3", running natively at 1920x1080p) the same issues apply whether in windowed or no-windowed mode. In fact, apart from what it says in the config.ini file I can't tell the difference between the two - I have the same small window opened in the middle of the screen either way.
In all cases DPI is set to 100%.
Playing the game, there is always only one board's width plus one hex (same in the editor) and although the log window is full size to begin with, as soon as the first Admin segment is entered it suddenly jumps down to half size, making it too small to be practical. In the editor, the buttons to change covered arc and turret of an AFV are still hidden behind the OOB window. It's becoming hard for me to see what about my own setup is at fault.
The game is very enjoyable, so long as I am only playing very small scenarios where it's manageable, despite its faults, but any of the larger scenarios are just impossible.
I tried playing Popel's attack, but it's excruciating. An enemy unit moves, apparently I have a line of sight to it, but the game does not tell me which of my units has a line of sight and so I have to scroll to the other and of the screen, select a unit, and then scroll all the way back again to see if it has a LOS. And then I have to repeat this multiple times until I find the unit with a LOS. Having done that, the enemy unit moves another hex, and I have to repeat this tedious process all over again. That's bad enough, but the enemy has many other units, for which I am going to have to repeat this again, for every single one of them, for every single hex of movement.
Where's the fun in that? There really isn't any.