It's in the game config file.
The setting is ("r.ScreenPercentage"), (Value=175,bModified=True))
I set mine to 175 which is 1.75. Doesn't look blurry but it doesn't hurt my performance. I'm running a 2070 and I set it to 1.25 and it looked so blurry but 2.00 was unbearably laggy so I just altered the setting in config.
Hope this helped.
I second that. It would be really nice. Even though we can modify the .ini file, I still believe it's not how it should be. It would be cool to type whatever value we wish to have. I could finally use the borderless windowed mode in a lesser resolution than 4K, which my GPU can't handle well.