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Solution first, explanations later.
This is for nVidia users, but I guess ATI users might have the same options somewhere.

1. In the game installation folder there's a file named "Config.exe", run it
2. Select "Full screen" (and disable movies? I dont know, I disabled them) and click OK
3. Open nVidia control panel
4. In the left pane select "Adjust desktop size and position"
5. In the right pane select "Aspect ratio"
6. In the "Perform scaling on" dropdown menu select "GPU"
7. Tick the "Override scaling mode set by games and programs" option (set it ON)
8. Apply
9. Run the game from GoG's client OR by making a direct link to "Sacred.exe", DO NOT use the default link created during game installation (for some reason the game stops responding even before showing the initial menu if you do)

!! IMPORTANT !!
If you have a graphic tablet (perhaps it happens only with Wacom ones?), unplug it before setting the desktop options or they will not stick and revert to default values as soon as you apply them.

IN CASE YOU INADVERTENTLY USE GOG'S LINK TO START THE GAME:
Your screen might become white or black and nothing more will happen.
Hit [Ctrl+Shift+Esc], this will open Windows' task manager allowing you to kill the process. If the usual "the process stopped responding" window appears just tell it to close the application.
You might experience an unresponsive mouse cursor, no panic, just give the process some seconds to close down and everything will go back to normal.

EXPLANATION ON WHY I MADE THIS POST:
I wanted to give this game another go, but after a bit I got tired of that very small patch in the middle of the monitor, I wanted more freedom to move around with my mouse.
So I googled around for a while, but never finding a definitive solution, only pieces of information here and there. Some would even involve 3rd party softwares, which I hate.
So I put together all I had collected and started trying. For some reason the nVidia control panel options wouldn't stick, so I googled more and took me a frustratingly long time to find that deeply buried thread reply mentioning graphic tablets.
I like to spare other people my same pains.

For me it works like a charm, not a single problem and I don't even have to revert to default nVidia options to play modern games (i.e.: Far Cry 5!!).
I still haven't plugged back in my graphic tablet, so I can't tell whether the desktop options would stick. Keep an eye on them, in case.
Post edited June 03, 2018 by Zehryo