Also having this issue. Game starts in a resolution that makes it larger than my screen and it doesn't seem to respond to mouse, keyboard or controller in any noticeable way.

I have a 4k screen and it is also appearing way bigger than the screen. I see maybe 1/4 of it.

hitting ALT+ENTER to window and un-window worked for me. Found in BUG: Resolution thread from Nov

right click on the file PlanetAlpha-Win64-Shipping.exe and set HIGH DPI SCALING to APPLICATION
[img]https://img.dtcn.com/image/digitaltrends/windows-10-manually-change-dpi-scaling-768x768.jpg[/img]

also open %LOCALAPPDATA%\PLANETALPHA\Saved\Config\WindowsNoEditor\GameUserSettings.ini and enter the following settings:
[/script/engine.gameusersettings]
FullscreenMode=1
DesiredScreenWidth=1920
DesiredScreenHeight=1080
bUseDynamicResolution=False
ResolutionSizeX=1920
ResolutionSizeY=1080

thanks,
worked like a charm!

I found the ALT+ENTER worked fine thanks.. BUT, the game judders all the time, it is so slow it is painful. Any idea how to fix this please?

did you check the FPS?
It needs some decent GPU. i played it some time ago on GeForce 2070 Super and it was smooth with 60+ FPS