Water was sprayed on my 19" monitor and damaged some parts of it. I need a solution that will let me specify the screen area to be used by Windows, as if it was a smaller monitor. Its default menu doesn't have this option.
Winsplit-Revolution might do what you need.
What model is your monitor? It may have a "1:1 pixel mode" or something like that in the settings. With that option set, you can specify a lower screen resolution on the computer, and the monitor will only use that number of pixels, instead of scaling the image to full screen.