So I downloaded the Belzebub mod last night and found that the game resolution was much bigger than what my settings indicated, and that fullscreen just did not work. I spent a bit of time solving this problem.
The solution that worked for me was the following:
1. Right-click Belzebub.exe
![Image](https://i.imgur.com/OS0aSv1.png)
2. Click "Properties" at the bottom, then click the "Compatibility" tab
![Image](https://i.imgur.com/4rKeKvs.png)
3. Check the box labeled "Disable fullscreen optimizations" (this fixes fullscreen)
![Image](https://i.imgur.com/37yXy9k.png)
4. Click the "Change high DPI settings" button, and check the box labeled "Override high DPI scaling behavior" and then click "OK" (this fixes resolution)
![Image](https://i.imgur.com/8RgeeHp.png)
5. Click "Apply" and the game will be fixed
Hope this helps someone!