Should work for everyone:
- Try holding Alt whilst the program is opening (white/black screen)
Windows:
- Whilst the program is open select it in the taskbar then press Windows key + an arrow key
- Whilst the program is open select it in the taskbar and press Alt+Space and select "Maximize" from the menu that should appear in the top left of your screen
- Go to C:\Users\<user>\AppData\LocalLow\BDcraft\Cubik and edit cubikdata.json so that "windowX" and "windowY" are 0
- Whilst the program is open right click it in the taskbar and select "Maximize"
- Go to ~/.config/unity3d/BDcraft/Cubik/ and edit cubikdata.json so that "windowX" and "windowY" are 0
I'm unsure about Mac as I have no access to one, but I suspect it's very similar to Linux (in the very least you can search for cubikdata.json and edit the variables as stated above).