how do I fix this? I'm using windows 10

Try changing the resolution in the game then restart.

And how do you do that when you can only see the top-left quarter of the screen?

It cannot be changed in-game, you need to use a launcher like Knossos.

I had this issue also, fixed it by changing the display scaling to 100% in the Windows 10 dispay settings menu. The default value is 150% so you just need to drag the lever down to 100% sign out and then back in again. There's a post on the main forum page saying the same thing. It's actually where I got the fix from.

Another way to fix it is to just change your Windows resolution to 1920x1080 (or something similar) before launching the game. Your error happens when your Windows resolution is very high, like 4k.